Add add_generation_prompt argument to apply_chat_template (#26573)

* Add add_generation_prompt argument to apply_chat_template

* Add add_generation_prompt argument to apply_chat_template and update default templates

* Fix typo

* Add generation prompts section to chat templating guide

* Add generation prompts section to chat templating guide

* Minor style fix

docs/source/en/chat_templating.md

@@ -218,10 +218,11 @@ input formats. Our default template for models that don't have a class-specific
 {% endfor %}
 ```
 
-If you like this one, here it is in one-liner form, ready to copy into your code:
+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!
 
 ```
-tokenizer.chat_template = "{% for message in messages %}{{'<|im_start|>' + message['role'] + '\n' + message['content'] + '<|im_end|>' + '\n'}}{% endfor %}"
+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 %})"
 ```
 
 This template wraps each message in `<|im_start|>` and `<|im_end|>` tokens, and simply writes the role as a string, which
@@ -240,6 +241,56 @@ The "user", "system" and "assistant" roles are the standard for chat, and we rec
 particularly if you want your model to operate well with [`ConversationalPipeline`]. However, you are not limited
 to these roles - templating is extremely flexible, and any string can be a role.
 
+## What are "generation prompts"?
+
+You may notice that the `apply_chat_template` method has an `add_generation_prompt` argument. This argument tells
+the template to add tokens that indicate the start of a bot response. For example, consider the following chat:
+
+```python
+messages = [
+    {"role": "user", "content": "Hi there!"},
+    {"role": "assistant", "content": "Nice to meet you!"},
+    {"role": "user", "content": "Can I ask a question?"}
+]
+```
+
+Here's what this will look like without a generation prompt, using the ChatML template we described above:
+
+```python
+>> tokenizer.apply_chat_template(messages, tokenize=False, add_generation_prompt=False)
+"""<|im_start|>user
+Hi there!<|im_end|>
+<|im_start|>assistant
+Nice to meet you!<|im_end|>
+<|im_start|>user
+Can I ask a question?<|im_end|>
+"""
+```
+
+And here's what it looks like **with** a generation prompt:
+
+```python
+>> tokenizer.apply_chat_template(messages, tokenize=False, add_generation_prompt=True)
+"""<|im_start|>user
+Hi there!<|im_end|>
+<|im_start|>assistant
+Nice to meet you!<|im_end|>
+<|im_start|>user
+Can I ask a question?<|im_end|>
+<|im_start|>assistant
+"""
+```
+
+Note that this time, we've added the tokens that indicate the start of a bot response. This ensures that when the model
+generates text it will write a bot response instead of doing something unexpected, like continuing the user's 
+message. Remember, chat models are still just language models - they're trained to continue text, and chat is just a 
+special kind of text to them! You need to guide them with the appropriate control tokens so they know what they're 
+supposed to be doing.
+
+Not all models require generation prompts. Some models, like BlenderBot and LLaMA, don't have any
+special tokens before bot responses. In these cases, the `add_generation_prompt` argument will have no effect. The exact
+effect that `add_generation_prompt` has will depend on the template being used.
+
 ## I want to use chat templates! How should I get started?
 
 If you have any chat models, you should set their `tokenizer.chat_template` attribute and test it using