[Bugfix] Fix mypy error with Protocol

[tjtanaa]

PLEASE FILL IN THE PR DESCRIPTION HERE ENSURING ALL CHECKLIST ITEMS (AT THE BOTTOM) HAVE BEEN CONSIDERED.

Purpose

@JartX I tested using other backend. This will cause issue as

FlashAttentionMetadata is not a generic class.

TreeAttentionMetadata, AiterFlashAttentionMetadata, TritonAttentionMetadata and FlashAttentionMetadata are 4 different instances.

Example error when using AITERFlashAttention Backend

(VllmWorker pid=175050) ERROR 08-20 09:55:33 [multiproc_executor.py:596]     assert isinstance(attn_metadata, FlashAttentionMetadata)                                                                                                                                         
(VllmWorker pid=175050) ERROR 08-20 09:55:33 [multiproc_executor.py:596] AssertionError     

Test Plan

Test Result

(Optional) Documentation Update

---

Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.

BEFORE SUBMITTING, PLEASE READ https://docs.vllm.ai/en/latest/contributing (anything written below this line will be removed by GitHub Actions)