[2/N] Pass model_config to the Attention constructors

[MatthewBonanni]

Purpose

We already pass cache_config and quant_config as arguments to Attention.__init__(), but model_config is routinely grabbed from get_current_vllm_config. #38124 requires accesssing model_config.dtype much more frequently, so this PR passes model_config as an argument to standardize and reduce reliance on get_current_vllm_config.

Test Plan

CI

Test Result

TBD

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• 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
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[mergify]

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[gemini-code-assist]

Code Review

This pull request updates the model executor layers and attention mechanisms across various model implementations to explicitly pass the model_config object. It also introduces a mapping from torch.dtype to KV cache string representations in vllm/utils/torch_utils.py to ensure correct cache configuration when cache_config is not provided. I have no further feedback to provide.