[GPUModelRunner] Refactor initialize_kv_cache

[heheda12345]

Purpose

Clean up initialize_kv_cache after the fast development of various attention-related features in the previous months to simplify the code and dependency between steps. It mainly touches the logic of

1. kv sharing
2. encoder only
3. hybrid mamba

Key modifications:

1. Some steps can be done after model loading. Move them to the end of load_models. [may needs some discussion]
2. Many operation (e.g., compute kernel_block_size) only needs the AttentionBackend type of each layer. We don't have to do them after builder is initialized.
3. kernel_block_size now includes EncoderOnlyAttention
4. simplify attn_group split logic
5. start to move some logic into utils so that it can be used in ModelRunnerV2 in the future.

I can split the above items into separate PRs if needed

depends on
#27929
#27753

Test Plan

Test Result

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[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @heheda12345.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @heheda12345.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork