Fix trtllm_ar failure (#1423)

Fix `TypeError: cannot be converted to pointer` error in AR+RMSNorm.

This was broken by

85d75ca

<!-- .github/pull_request_template.md -->

## 📌 Description

<!-- What does this PR do? Briefly describe the changes and why they’re
needed. -->

## 🔍 Related Issues

<!-- Link any related issues here -->

## 🚀 Pull Request Checklist

Thank you for contributing to FlashInfer! Before we review your pull
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.

> If you are unsure about how to set up `pre-commit`, see [the
pre-commit documentation](https://pre-commit.com/).

## 🧪 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.
-->

Signed-off-by: Po-Han Huang <[email protected]>

Author: nvpohanh

flashinfer/comm/trtllm_ar.py

@@ -16,7 +16,7 @@
 
 import functools
 import logging
-from ctypes import c_void_p
+from ctypes import c_void_p, cast
 from types import SimpleNamespace
 from typing import List, Optional, Tuple, Union
 
@@ -610,7 +610,7 @@ def trtllm_create_ipc_workspace_for_all_reduce_fusion(
     # Set flag_ptr[3] = lamport_comm_size
     lamport_comm_size_bytes = lamport_comm_size.to_bytes(4, byteorder="little")
     cudart.cudaMemcpy(
-        c_void_p(flag_ptr.value + 3 * 4), c_void_p(lamport_comm_size_bytes), 4
+        c_void_p(flag_ptr.value + 3 * 4), cast(lamport_comm_size_bytes, c_void_p), 4
     )
     print("set flag_ptr[3] = lamport_comm_size: ", lamport_comm_size)
     # add flag_ptr to workspace