Reset GPU tokens before reuse (#795)

Fixes a token-reuse bug in `TokenPool` that's independent of MNNVL.

## Bug

`TokenPool` hands out 8-byte device-memory slots used as
device-semaphore counters. The deleter only cleared the bitmap — the
underlying GPU memory was left as-is. When a token was freed and later
re-allocated, the new semaphore inherited the previous counter value
instead of starting at 0, breaking subsequent `signal()/wait()` math.

## Fix

* Add a synchronous `gpuMemset` host helper (mirrors `gpuMemcpy` /
`gpuMemcpyAsync`).
* Zero the slot inside the `TokenPool` deleter so recycled tokens hand
out a clean counter. The very-first allocation is already zeroed by
`gpuCallocPhysical` (`src/core/gpu_utils.cc:227-228`), so first-time
tokens are also clean — the deleter only has to handle the recycle case.

## Notes

* Public wrapper is named `mscclpp::gpuMemset` (not `mscclpp::memset`)
for symmetry with `gpuMemcpy` and to avoid shadowing `std::memset` in
TUs that pull the namespace in.
* Zeroing happens on release rather than acquire so the cost is paid in
the typically less perf-sensitive teardown path.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/core/gpu_utils.cc

@@ -267,6 +267,13 @@ void gpuMemcpy(void* dst, const void* src, size_t bytes, cudaMemcpyKind kind) {
   MSCCLPP_CUDATHROW(cudaStreamSynchronize(stream));
 }
 
+void gpuMemset(void* ptr, int value, size_t bytes) {
+  AvoidCudaGraphCaptureGuard cgcGuard;
+  CudaStreamWithFlags stream(cudaStreamNonBlocking);
+  MSCCLPP_CUDATHROW(cudaMemsetAsync(ptr, value, bytes, stream));
+  MSCCLPP_CUDATHROW(cudaStreamSynchronize(stream));
+}
+
 }  // namespace detail
 
 bool isNvlsSupported() {

src/core/utils_internal.cc

@@ -248,6 +248,9 @@ TokenPool::TokenPool(size_t nToken) : nToken_(nToken) {
 
 std::shared_ptr<uint64_t> TokenPool::getToken() {
   auto deleter = [self = shared_from_this()](uint64_t* token) {
+    // Zero the slot on release so the next allocator hands out a clean
+    // semaphore counter (matches a freshly-allocated slot).
+    mscclpp::gpuMemset(token, 0, sizeof(uint64_t));
     size_t index = (token - self->baseAddr_) / UINT64_WIDTH;
     size_t bit = (token - self->baseAddr_) % UINT64_WIDTH;
     uint64_t mask = 1UL << bit;