Feat: Add support for tokenizer’s or custom jinja chat_template (#1970)

* Allow using tokenizer's default chat template with fallbacks

Summary of changes:

1. Adds `tokenizer_default` as option for `chat_template` in
   `chat_template` prompt strategy that allows using the chat template
   from tokenizer's config.json
2. Allows falling back to chat templates available in axolotl if
   tokenizer does not have a chat template
3. Adds a mistral chat template which supports system message - taken
   from https://github.com/chujiezheng/chat_templates/blob/main/chat_templates/mistral-instruct.jinja

---

Why?

Many popular models are not trained with chatml format. As a result for
the model to correctly learn chatml we have to turn on train_on_inputs
which requires more compute and time. If we can use the model's already
learned chat template we can just learn the output tokens

---

Todo:

- Write tests

* Add tests

* Fix lint and bug post merge from main

* Add option `chat_template_jinja` to provide a jinja template

* remove custom mistral template

* Address review comments and add docs

* Update docs/dataset-formats/conversation.qmd

Co-authored-by: NanoCode012 <kevinvong@rocketmail.com>

* fix: set default to tokenizer template

* Merge branch 'main' into cj_tokenizer_default_prompt_template

* chore: remove redundant function

* fix: re-arrange enum declaration position

* fix: refactor artifact left from main merge

* feat(doc): updated config with chat template options and clarified examples

* chore: clarify doc

* chore: added example for non-default template

* chore: refactor

* fix: test

* fix: config being dropped and unittest to catch that

* chore: lint

* chore: skip duplicate

* fix: rename var after merge

* feat: add test for levy's dpo case

* fix: remove default setting on edge case where chat template overriden in dataset section

* feat: handle sharegpt deprecation better in docs

* feat: add example using fallback

* feat: handles chat_template requiring specific user/assistant order

* fix: update test based on new defaults

* fix: imported name incorrectly updated on merge

* chore: lint

* fix: update dummy message to prevent potential overlap with real content

* fix(doc): formatting

* fix: update bradleyterry to use new chat_template

---------

Co-authored-by: Chirag Jain <jain.chirag925@gmail.com>

README.md

@@ -383,7 +383,7 @@ See [examples](examples) for quick start. It is recommended to duplicate and mod
         - typescript
       type: ... # unimplemented custom format
 
-      # fastchat conversation (deprecation soon, use chat_template)
+      # fastchat conversation (deprecation soon, use chat_template https://axolotl-ai-cloud.github.io/axolotl/docs/dataset-formats/conversation.html#chat_template)
       # See 'conversation' options: https://github.com/lm-sys/FastChat/blob/main/fastchat/conversation.py
     - path: ...
       type: sharegpt