Add chat template

This PR adds a [chat template](https://huggingface.co/docs/transformers/main/chat_templating) following the ChatML format, which means users can just use `tokenizer.apply_chat_template` to handle chat formatting for them.

tokenizer_config.json

@@ -45,6 +45,7 @@
   },
   "additional_special_tokens": [],
   "bos_token": "<s>",
+  "chat_template": "{% if not add_generation_prompt is defined %}{% set add_generation_prompt = false %}{% endif %}{% for message in messages %}{{'<|im_start|>' + message['role'] + '\n' + message['content'] + '<|im_end|>' + '\n'}}{% endfor %}{% if add_generation_prompt %}{{ '<|im_start|>assistant\n' }}{% endif %})",
   "clean_up_tokenization_spaces": false,
   "eos_token": "<|im_end|>",
   "legacy": true,