[torch] Address -Wreturn-type warning when compiling for AMD (pytorch#138951)

Summary: Yep yep see title

Test Plan: CI

Differential Revision: D64971115

Pull Request resolved: pytorch#138951
Approved by: https://github.com/cyyever, https://github.com/adamomainz

Author: danzimm

torch/csrc/distributed/c10d/CUDASymmetricMemory-inl.h

@@ -59,6 +59,7 @@ __device__ __forceinline__ uint32_t
 cas(uint32_t* addr, uint32_t compare, uint32_t val) {
 #if defined(USE_ROCM) || (defined(__CUDA_ARCH__) && (__CUDA_ARCH__ < 800))
   CUDA_KERNEL_ASSERT(false);
+  return 0;
 #else
   uint32_t old_val;
   if constexpr (Sem == MemOpSem::Relaxed) {