captum/attr/_core/integrated_gradients.py

#!/usr/bin/env python3
import typing
from typing import Any, Callable, List, Tuple, Union

import torch
from captum._utils.common import (
    _expand_additional_forward_args,
    _expand_target,
    _format_additional_forward_args,
    _format_output,
    _is_tuple,
)
from captum._utils.typing import (
    BaselineType,
    Literal,
    TargetType,
    TensorOrTupleOfTensorsGeneric,
)
from captum.attr._utils.approximation_methods import approximation_parameters
from captum.attr._utils.attribution import GradientAttribution
from captum.attr._utils.batching import _batch_attribution
from captum.attr._utils.common import (
    _format_input_baseline,
    _reshape_and_sum,
    _validate_input,
)
from captum.log import log_usage
from torch import Tensor


class IntegratedGradients(GradientAttribution):
    r"""
    Integrated Gradients is an axiomatic model interpretability algorithm that
    assigns an importance score to each input feature by approximating the
    integral of gradients of the model's output with respect to the inputs
    along the path (straight line) from given baselines / references to inputs.

    Baselines can be provided as input arguments to attribute method.
    To approximate the integral we can choose to use either a variant of
    Riemann sum or Gauss-Legendre quadrature rule.

    More details regarding the integrated gradients method can be found in the
    original paper:
    https://arxiv.org/abs/1703.01365

    """

    def __init__(
        self,
        forward_func: Callable,
        multiply_by_inputs: bool = True,
    ) -> None:
        r"""
        Args:

            forward_func (callable):  The forward function of the model or any
                    modification of it
            multiply_by_inputs (bool, optional): Indicates whether to factor
                    model inputs' multiplier in the final attribution scores.
                    In the literature this is also known as local vs global
                    attribution. If inputs' multiplier isn't factored in,
                    then that type of attribution method is also called local
                    attribution. If it is, then that type of attribution
                    method is called global.
                    More detailed can be found here:
                    https://arxiv.org/abs/1711.06104

                    In case of integrated gradients, if `multiply_by_inputs`
                    is set to True, final sensitivity scores are being multiplied by
                    (inputs - baselines).
        """
        GradientAttribution.__init__(self, forward_func)
        self._multiply_by_inputs = multiply_by_inputs

    # The following overloaded method signatures correspond to the case where
    # return_convergence_delta is False, then only attributions are returned,
    # and when return_convergence_delta is True, the return type is
    # a tuple with both attributions and deltas.
    @typing.overload
    def attribute(
        self,
        inputs: TensorOrTupleOfTensorsGeneric,
        baselines: BaselineType = None,
        target: TargetType = None,
        additional_forward_args: Any = None,
        n_steps: int = 50,
        method: str = "gausslegendre",
        internal_batch_size: Union[None, int] = None,
        return_convergence_delta: Literal[False] = False,
    ) -> TensorOrTupleOfTensorsGeneric:
        ...

    @typing.overload
    def attribute(
        self,
        inputs: TensorOrTupleOfTensorsGeneric,
        baselines: BaselineType = None,
        target: TargetType = None,
        additional_forward_args: Any = None,
        n_steps: int = 50,
        method: str = "gausslegendre",
        internal_batch_size: Union[None, int] = None,
        *,
        return_convergence_delta: Literal[True],
    ) -> Tuple[TensorOrTupleOfTensorsGeneric, Tensor]:
        ...

    @log_usage()
    def attribute(  # type: ignore
        self,
        inputs: TensorOrTupleOfTensorsGeneric,
        baselines: BaselineType = None,
        target: TargetType = None,
        additional_forward_args: Any = None,
        n_steps: int = 50,
        method: str = "gausslegendre",
        internal_batch_size: Union[None, int] = None,
        return_convergence_delta: bool = False,
    ) -> Union[
        TensorOrTupleOfTensorsGeneric, Tuple[TensorOrTupleOfTensorsGeneric, Tensor]
    ]:
        r"""
        This method attributes the output of the model with given target index
        (in case it is provided, otherwise it assumes that output is a
        scalar) to the inputs of the model using the approach described above.

        In addition to that it also returns, if `return_convergence_delta` is
        set to True, integral approximation delta based on the completeness
        property of integrated gradients.

        Args:

            inputs (tensor or tuple of tensors):  Input for which integrated
                        gradients are computed. If forward_func takes a single
                        tensor as input, a single input tensor should be provided.
                        If forward_func takes multiple tensors as input, a tuple
                        of the input tensors should be provided. It is assumed
                        that for all given input tensors, dimension 0 corresponds
                        to the number of examples, and if multiple input tensors
                        are provided, the examples must be aligned appropriately.
            baselines (scalar, tensor, tuple of scalars or tensors, optional):
                        Baselines define the starting point from which integral
                        is computed and can be provided as:

                        - a single tensor, if inputs is a single tensor, with
                          exactly the same dimensions as inputs or the first
                          dimension is one and the remaining dimensions match
                          with inputs.

                        - a single scalar, if inputs is a single tensor, which will
                          be broadcasted for each input value in input tensor.

                        - a tuple of tensors or scalars, the baseline corresponding
                          to each tensor in the inputs' tuple can be:

                          - either a tensor with matching dimensions to
                            corresponding tensor in the inputs' tuple
                            or the first dimension is one and the remaining
                            dimensions match with the corresponding
                            input tensor.

                          - or a scalar, corresponding to a tensor in the
                            inputs' tuple. This scalar value is broadcasted
                            for corresponding input tensor.
                        In the cases when `baselines` is not provided, we internally
                        use zero scalar corresponding to each input tensor.

                        Default: None
            target (int, tuple, tensor or list, optional):  Output indices for
                        which gradients are computed (for classification cases,
                        this is usually the target class).
                        If the network returns a scalar value per example,
                        no target index is necessary.
                        For general 2D outputs, targets can be either:

                        - a single integer or a tensor containing a single
                          integer, which is applied to all input examples

                        - a list of integers or a 1D tensor, with length matching
                          the number of examples in inputs (dim 0). Each integer
                          is applied as the target for the corresponding example.

                        For outputs with > 2 dimensions, targets can be either:

                        - A single tuple, which contains #output_dims - 1
                          elements. This target index is applied to all examples.

                        - A list of tuples with length equal to the number of
                          examples in inputs (dim 0), and each tuple containing
                          #output_dims - 1 elements. Each tuple is applied as the
                          target for the corresponding example.

                        Default: None
            additional_forward_args (any, optional): If the forward function
                        requires additional arguments other than the inputs for
                        which attributions should not be computed, this argument
                        can be provided. It must be either a single additional
                        argument of a Tensor or arbitrary (non-tuple) type or a
                        tuple containing multiple additional arguments including
                        tensors or any arbitrary python types. These arguments
                        are provided to forward_func in order following the
                        arguments in inputs.
                        For a tensor, the first dimension of the tensor must
                        correspond to the number of examples. It will be
                        repeated for each of `n_steps` along the integrated
                        path. For all other types, the given argument is used
                        for all forward evaluations.
                        Note that attributions are not computed with respect
                        to these arguments.
                        Default: None
            n_steps (int, optional): The number of steps used by the approximation
                        method. Default: 50.
            method (string, optional): Method for approximating the integral,
                        one of `riemann_right`, `riemann_left`, `riemann_middle`,
                        `riemann_trapezoid` or `gausslegendre`.
                        Default: `gausslegendre` if no method is provided.
            internal_batch_size (int, optional): Divides total #steps * #examples
                        data points into chunks of size at most internal_batch_size,
                        which are computed (forward / backward passes)
                        sequentially. internal_batch_size must be at least equal to
                        #examples.
                        For DataParallel models, each batch is split among the
                        available devices, so evaluations on each available
                        device contain internal_batch_size / num_devices examples.
                        If internal_batch_size is None, then all evaluations are
                        processed in one batch.
                        Default: None
            return_convergence_delta (bool, optional): Indicates whether to return
                    convergence delta or not. If `return_convergence_delta`
                    is set to True convergence delta will be returned in
                    a tuple following attributions.
                    Default: False
        Returns:
            **attributions** or 2-element tuple of **attributions**, **delta**:
            - **attributions** (*tensor* or tuple of *tensors*):
                    Integrated gradients with respect to each input feature.
                    attributions will always be the same size as the provided
                    inputs, with each value providing the attribution of the
                    corresponding input index.
                    If a single tensor is provided as inputs, a single tensor is
                    returned. If a tuple is provided for inputs, a tuple of
                    corresponding sized tensors is returned.
            - **delta** (*tensor*, returned if return_convergence_delta=True):
                    The difference between the total approximated and true
                    integrated gradients. This is computed using the property
                    that the total sum of forward_func(inputs) -
                    forward_func(baselines) must equal the total sum of the
                    integrated gradient.
                    Delta is calculated per example, meaning that the number of
                    elements in returned delta tensor is equal to the number of
                    of examples in inputs.

        Examples::

            >>> # ImageClassifier takes a single input tensor of images Nx3x32x32,
            >>> # and returns an Nx10 tensor of class probabilities.
            >>> net = ImageClassifier()
            >>> ig = IntegratedGradients(net)
            >>> input = torch.randn(2, 3, 32, 32, requires_grad=True)
            >>> # Computes integrated gradients for class 3.
            >>> attribution = ig.attribute(input, target=3)
        """
        # Keeps track whether original input is a tuple or not before
        # converting it into a tuple.
        is_inputs_tuple = _is_tuple(inputs)

        inputs, baselines = _format_input_baseline(inputs, baselines)

        _validate_input(inputs, baselines, n_steps, method)

        if internal_batch_size is not None:
            num_examples = inputs[0].shape[0]
            attributions = _batch_attribution(
                self,
                num_examples,
                internal_batch_size,
                n_steps,
                inputs=inputs,
                baselines=baselines,
                target=target,
                additional_forward_args=additional_forward_args,
                method=method,
            )
        else:
            attributions = self._attribute(
                inputs=inputs,
                baselines=baselines,
                target=target,
                additional_forward_args=additional_forward_args,
                n_steps=n_steps,
                method=method,
            )

        if return_convergence_delta:
            start_point, end_point = baselines, inputs
            # computes approximation error based on the completeness axiom
            delta = self.compute_convergence_delta(
                attributions,
                start_point,
                end_point,
                additional_forward_args=additional_forward_args,
                target=target,
            )
            return _format_output(is_inputs_tuple, attributions), delta
        return _format_output(is_inputs_tuple, attributions)

    def _attribute(
        self,
        inputs: Tuple[Tensor, ...],
        baselines: Tuple[Union[Tensor, int, float], ...],
        target: TargetType = None,
        additional_forward_args: Any = None,
        n_steps: int = 50,
        method: str = "gausslegendre",
        step_sizes_and_alphas: Union[None, Tuple[List[float], List[float]]] = None,
    ) -> Tuple[Tensor, ...]:
        if step_sizes_and_alphas is None:
            # retrieve step size and scaling factor for specified
            # approximation method
            step_sizes_func, alphas_func = approximation_parameters(method)
            step_sizes, alphas = step_sizes_func(n_steps), alphas_func(n_steps)
        else:
            step_sizes, alphas = step_sizes_and_alphas

        # scale features and compute gradients. (batch size is abbreviated as bsz)
        # scaled_features' dim -> (bsz * #steps x inputs[0].shape[1:], ...)
        scaled_features_tpl = tuple(
            torch.cat(
                [baseline + alpha * (input - baseline) for alpha in alphas], dim=0
            ).requires_grad_()
            for input, baseline in zip(inputs, baselines)
        )

        additional_forward_args = _format_additional_forward_args(
            additional_forward_args
        )
        # apply number of steps to additional forward args
        # currently, number of steps is applied only to additional forward arguments
        # that are nd-tensors. It is assumed that the first dimension is
        # the number of batches.
        # dim -> (bsz * #steps x additional_forward_args[0].shape[1:], ...)
        input_additional_args = (
            _expand_additional_forward_args(additional_forward_args, n_steps)
            if additional_forward_args is not None
            else None
        )
        expanded_target = _expand_target(target, n_steps)

        # grads: dim -> (bsz * #steps x inputs[0].shape[1:], ...)
        grads = self.gradient_func(
            forward_fn=self.forward_func,
            inputs=scaled_features_tpl,
            target_ind=expanded_target,
            additional_forward_args=input_additional_args,
        )

        # flattening grads so that we can multilpy it with step-size
        # calling contiguous to avoid `memory whole` problems
        scaled_grads = [
            grad.contiguous().view(n_steps, -1)
            * torch.tensor(step_sizes).view(n_steps, 1).to(grad.device)
            for grad in grads
        ]

        # aggregates across all steps for each tensor in the input tuple
        # total_grads has the same dimensionality as inputs
        total_grads = tuple(
            _reshape_and_sum(
                scaled_grad, n_steps, grad.shape[0] // n_steps, grad.shape[1:]
            )
            for (scaled_grad, grad) in zip(scaled_grads, grads)
        )

        # computes attribution for each tensor in input tuple
        # attributions has the same dimensionality as inputs
        if not self.multiplies_by_inputs:
            attributions = total_grads
        else:
            attributions = tuple(
                total_grad * (input - baseline)
                for total_grad, input, baseline in zip(total_grads, inputs, baselines)
            )
        return attributions

    def has_convergence_delta(self) -> bool:
        return True

    @property
    def multiplies_by_inputs(self):
        return self._multiply_by_inputs