[1/N][Refactor] Remove the custom split_qkv

[gcanlin]

What this PR does / why we need it?

Due to vLLM PR #23190, I think we no longer need to worry about the additional communication overhead caused by TP. Now, we can simply set --mm-encoder-tp-mode to enable encoder-side DP. Specifically, in split_qkv, tp_size will be equal to 1, so all_gather_interleave will not be triggered.

I will complete it tomorrow.

Does this PR introduce any user-facing change?

How was this patch tested?

• vLLM version: v0.11.0
• vLLM main: vllm-project/vllm@83f478b

[github-actions]

👋 Hi! Thank you for contributing to the vLLM Ascend project. The following points will speed up your PR merge:‌‌

• A PR should do only one thing, smaller PRs enable faster reviews.
• Every PR should include unit tests and end-to-end tests ‌to ensure it works and is not broken by other future PRs.
• Write the commit message by fulfilling the PR description to help reviewer and future developers understand.

If CI fails, you can run linting and testing checks locally according Contributing and Testing.