Context

What is the purpose of this PR? Is it to

• add a new feature
• fix a bug
• update tests and/or documentation
• other (please add here)

Please link to any issues this PR addresses.

This adds a conversation_style of openai, similar to the openai one by @musabgultekin in #824 (comment).

Changelog

What are the changes made in this PR?

• Add conversion function of openai_to_llama2_messages

It now supports converting sample like (following OpenAI API) the following directly to internal llama2_messages.

{
    # key could be "messages" OR "conversations"
    "messages": [
        {
            "role": <system|user|assistant>,
            "content": <message>,
        },
        ...
    ]
}

Example usage in config:

# Dataset
dataset:
  _component_: torchtune.datasets.chat_dataset
  source: json
  data_files: data/datasets/codeact-mixture.jsonl
  split: train
  conversation_style: standard_chat
  chat_format: ChatMLFormat
  max_seq_len: 16384
seed: 42
shuffle: True

Test plan

Please make sure to do each of the following if applicable to your PR. (If you're not sure about any one of these just ask and we will happily help.)

• run pre-commit hooks and linters (make sure you've first installed via pre-commit install)
• add unit tests for any new functionality
• update docstrings for any new or updated methods or classes
• run unit tests via pytest tests
• run recipe tests via pytest tests -m integration_test
• manually run any new or modified recipes with sufficient proof of correctness
  • include relevant commands and any other artifacts in this summary (pastes of loss curves, eval results, etc.)