[BugFix][FlashInfer] Fix potential race condition for paged_kv_indptr_cpu

[WoosukKwon]

No description provided.

[gemini-code-assist]

Code Review

This pull request correctly identifies and fixes a potential race condition in the FlashInfer backend related to paged_kv_indptr_cpu. The race condition could occur during asynchronous data transfers to the GPU, especially when CUDA graphs are enabled. The implemented solution, which involves using an intermediate buffer (paged_kv_indptr_buffer) for the asynchronous copy, is a standard and effective way to resolve this kind of issue. The changes also include a minor optimization for calculating the number of actual pages. Overall, the fix is well-implemented and enhances the robustness of the attention mechanism.