Update model description

README.md

@@ -11,13 +11,9 @@ Disclaimer: The team releasing DETR did not write a model card for this model so
 
 ## Model description
 
-The DETR model is an encoder-decoder transformer with a convolutional backbone. 
+The DETR model is an encoder-decoder transformer with a convolutional backbone. Two heads are added on top of the decoder outputs in order to perform object detection: a linear layer for the class labels and a MLP (multi-layer perceptron) for the bounding boxes. The model uses so-called object queries to detect objects in an image. Each object query looks for a particular object in the image. For COCO, the number of object queries is set to 100. 
 
-First, an image is sent through a CNN backbone, outputting a lower-resolution feature map, typically of shape (1, 2048, height/32, width/32). This is then projected to match the hidden dimension of the Transformer, which is 256 by default, using a nn.Conv2D layer. Next, the feature map is flattened and transposed to obtain a tensor of shape (batch_size, seq_len, d_model) = (1, width/32*height/32, 256). 
-
-This is sent through the encoder, outputting encoder_hidden_states of the same shape. Next, so-called object queries are sent through the decoder. This is just a tensor of shape (batch_size, num_queries, d_model), with num_queries typically set to 100 and is initialized with zeros. Each object query looks for a particular object in the image. Next, the decoder updates these object queries through multiple self-attention and encoder-decoder attention layers to output decoder_hidden_states of the same shape: (batch_size, num_queries, d_model). Next, two heads are added on top for object detection: a linear layer for classifying each object query into one of the objects or "no object", and a MLP to predict bounding boxes for each query. So the number of queries actually determines the maximum number of objects the model can detect in an image.
-
-The model is trained using a "bipartite matching loss": so what we actually do is compare the predicted classes + bounding boxes of each of the N = 100 object queries to the ground truth annotations, padded up to the same length N (so if an image only contains 4 objects, 96 annotations will just have a "no object" as class and "no bounding box" as bounding box). The Hungarian matching algorithm is used to create an optimal one-to-one mapping between each of the N queries and each of the N annotations. Next, standard cross-entropy for the classes and L1 regression loss for the bounding boxes are used to optimize the parameters of the model.
+The model is trained using a "bipartite matching loss": one compares the predicted classes + bounding boxes of each of the N = 100 object queries to the ground truth annotations, padded up to the same length N (so if an image only contains 4 objects, 96 annotations will just have a "no object" as class and "no bounding box" as bounding box). The Hungarian matching algorithm is used to create an optimal one-to-one mapping between each of the N queries and each of the N annotations. Next, standard cross-entropy (for the classes) and a linear combination of the L1 and generalized IoU loss (for the bounding boxes) are used to optimize the parameters of the model.
 
 ## Intended uses & limitations