[c10d][nvshmem] fix missing override compilation error for nvshmem symmetric code (pytorch#159557)

Summary:
Fix error when compiling nvshmem code section `NVSHMEMSymmetricMemory.cu` with BUCK

```
fbcode/caffe2/torch/csrc/distributed/c10d/symm_mem/NVSHMEMSymmetricMemory.cu:154:20: error: 'get_buffer' overrides a member function but is not marked 'override' [-Werror,-Winconsistent-missing-override]
  154 | virtual at::Tensor get_buffer(int
      |                    ^
fbcode/caffe2/torch/csrc/distributed/c10d/symm_mem/SymmetricMemory.hpp:56:20: note: overridden virtual function is here
   56 | virtual at::Tensor get_buffer(int rank, c10::IntArrayRef sizes, c10::ScalarType dtype, int64_t storage_offset) = 0;
```

Test Plan:
Build test + CI

Rollback Plan:

Differential Revision: D78813586

Pull Request resolved: pytorch#159557
Approved by: https://github.com/kwen2501

Author: jushg

torch/csrc/distributed/c10d/symm_mem/NVSHMEMSymmetricMemory.cu

@@ -155,7 +155,7 @@ class NVSHMEMSymmetricMemory : public SymmetricMemory {
       int rank,
       c10::IntArrayRef sizes,
       c10::ScalarType dtype,
-      int64_t storage_offset) {
+      int64_t storage_offset) override {
     // TODO: deduplicate
     const size_t numel = std::accumulate(
         sizes.begin(),