[v1] Re-init input batch for multiple kv cache groups

[heheda12345]

Ideally, we should initialize the input batch inside initialize_kv_cache based on the kv cache config. However, as in #18298, due to some unknown reasons, we have to initialize the input batch before load_model, quantization + weight offloading will fail otherwise.

As a temporary solution, we initialize the input batch in GPUModelRunner.__init__, and re-initialize it in initialize_kv_cache only when necessary.

To avoid the error in weight offloading + quantization, we only allow the re-initialization when weight offloading is disabled. This is fine as there is only one kv cache group now, and after hybrid allocator is landed, we can disable hybrid allocator and ensure there is only one kv cache group in this case.

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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

[WoosukKwon]

LGTM