Remote const qualifier to avoid compilation error (#1421)

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## 📌 Description

Removing the const qualifier so that compiler could generate the copy
constructor.

## 🔍 Related Issues

Without the change, I'm seeing the following build error.

```
flashinfer/comm/vllm_custom_all_reduce.cuh:60:35: error: cannot initialize a parameter of type 'void *' with an rvalue of type 'const void *__restrict (*)[8]'
   60 | struct __attribute((aligned(16))) RankData { 
      |                                   ^~~~~~~~

...

flashinfer/comm/vllm_custom_all_reduce.cuh:407:26: note: in instantiation of member function 'std::vector<vllm::RankData>::vector' requested here
  407 | __T0::vector< RankData>  rank_data(num_buffers); 
      |                          ^

```

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request, please make sure the following items are complete.

### ✅ Pre-commit Checks

- [ ] I have installed `pre-commit` by running `pip install pre-commit`
(or used your preferred method).
- [ ] I have installed the hooks with `pre-commit install`.
- [ ] I have run the hooks manually with `pre-commit run --all-files`
and fixed any reported issues.

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## 🧪 Tests

- [ ] Tests have been added or updated as needed.
- [ ] All tests are passing (`unittest`, etc.).

## Reviewer Notes

<!-- Optional: anything you'd like reviewers to focus on, concerns, etc.
-->

Author: 842974287

include/flashinfer/comm/vllm_custom_all_reduce.cuh

@@ -58,7 +58,7 @@ struct Signal {
 };
 
 struct __align__(16) RankData {
-  const void* __restrict__ ptrs[8];
+  void* __restrict__ ptrs[8];
 };
 
 struct __align__(16) RankSignals {