Update chat template warnings/guides (#27634)

* Update default ChatML template

* Update docs/warnings

* Update docs/source/en/chat_templating.md
Co-authored-by: Arthur <48595927+ArthurZucker@users.noreply.github.com>

* Slight rework

---------
Co-authored-by: Arthur <48595927+ArthurZucker@users.noreply.github.com>

docs/source/en/chat_templating.md

@@ -376,7 +376,10 @@ input formats. Our default template for models that don't have a class-specific
 ```
 
 If you like this one, here it is in one-liner form, ready to copy into your code. The one-liner also includes
-handy support for "generation prompts" - see the next section for more!
+handy support for [generation prompts](#what-are-generation-prompts), but note that it doesn't add BOS or EOS tokens!
+If your model expects those, they won't be added automatically by `apply_chat_template` - in other words, the
+text will be tokenized with `add_special_tokens=False`. This is to avoid potential conflicts between the template and
+the `add_special_tokens` logic. If your model expects special tokens, make sure to add them to the template!
 
 ```
 tokenizer.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 %}"

src/transformers/tokenization_utils_base.py

@@ -1786,7 +1786,7 @@ class PreTrainedTokenizerBase(SpecialTokensMixin, PushToHubMixin):
         """
         logger.warning_once(
             "\nNo chat template is defined for this tokenizer - using a default chat template "
-            "that implements the ChatML format. If the default is not appropriate for "
+            "that implements the ChatML format (without BOS/EOS tokens!). If the default is not appropriate for "
             "your model, please set `tokenizer.chat_template` to an appropriate template. "
             "See https://huggingface.co/docs/transformers/main/chat_templating for more information.\n"
         )