[Docs] Write the Adding a New Model section

[WoosukKwon]

Closes #65
This PR adds the Adding a New Model section to the doc.

Because the process of adding a model is complex and highly depends on the model architecture, I only provided the high-level guidance. I think we can further improve this section later.

[zhuohan123]

We can provide an example for this doc:

Suggested change

This document provides a high-level guide on the process of adding a new model into CacheFlow.

This document provides a high-level guide on the process of adding a new model into CacheFlow. For example, how to add the `OPT model in huggingface <https://github.com/huggingface/transformers/blob/v4.29.1/src/transformers/models/opt/modeling_opt.py#L812>`_ to `CacheFlow <https://github.com/WoosukKwon/cacheflow/blob/62ec38ea4148bb8147f346f7e01cab6b8a2ec7b6/cacheflow/model_executor/models/opt.py#L248>`_

[WoosukKwon]

Changed to:

This document provides a high-level guide on integrating a `HuggingFace Transformers <https://github.com/huggingface/transformers>`_ model into CacheFlow.

I didn't add the example here because a similar example is provided in the section 1 (for llama).

[WoosukKwon]

@zhuohan123 I've replied to your comments and polished the writing. PTAL.

[zhuohan123]

Maybe mention the modified LLaMA model in CacheFlow (https://github.com/WoosukKwon/cacheflow/blob/main/cacheflow/model_executor/models/llama.py) here as a reference?

[WoosukKwon]

Added and changed the model to OPT.

[zhuohan123]

LGTM! My general comment is that we can have an actual code example. This can let the user directly compare the difference between the Huggingface model and the CacheFlow model. The current texts are very detailed but still a bit abstract. I initially suggested using OPT as such an example because we have many custom kernels for LLaMA. In this case, directly comparing Huggingface LLaMA and Cacheflow LLaMA can be difficult.

[WoosukKwon]

LGTM! My general comment is that we can have an actual code example. This can let the user directly compare the difference between the Huggingface model and the CacheFlow model. The current texts are very detailed but still a bit abstract. I initially suggested using OPT as such an example because we have many custom kernels for LLaMA. In this case, directly comparing Huggingface LLaMA and Cacheflow LLaMA can be difficult.

Agreed. I actually wanted to have a walk-through example, but found it too complicated to explain. Please feel free to add any example to the doc.