Use nullptr in faiss/gpu/StandardGpuResources.cpp (facebookresearch#4232)

Summary:
Pull Request resolved: facebookresearch#4232

`nullptr` is preferable to `0` or `NULL`. Let's use it everywhere so we can enable `-Wzero-as-null-pointer-constant`.

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Reviewed By: dtolnay

Differential Revision: D70818157

fbshipit-source-id: a46d64b6d80844f5246f7df236eb6ec54ce2886f

Author: r-barnes

faiss/gpu/StandardGpuResources.cpp

@@ -411,7 +411,7 @@ void StandardGpuResourcesImpl::initializeForDevice(int device) {
     raftHandles_.emplace(std::make_pair(device, defaultStream));
 #endif
 
-    cudaStream_t asyncCopyStream = 0;
+    cudaStream_t asyncCopyStream = nullptr;
     CUDA_VERIFY(
             cudaStreamCreateWithFlags(&asyncCopyStream, cudaStreamNonBlocking));