Fix C2 Ambiguous namespace (#89534)

Summary: cuda:: is a ambiguous namespace. Make it explicit c10::cuda

Differential Revision: D41469007
/caffe2/caffe2/core/context_gpu.cu(564): error: "caffe2::cuda" is ambiguous/caffe2/caffe2/core/context_gpu.cu(564): error: expected a ";"/caffe2/caffe2/core/context_gpu.cu(568): warning #12-D: parsing restarts here after previous syntax error
Remark: The warnings can be suppressed with "-diag-suppress <warning-number>"/caffe2/caffe2/core/context_gpu.cu(569): error: "caffe2::cuda" is ambiguous/caffe2/caffe2/core/context_gpu.cu(628): error: "caffe2::cuda" is ambiguous
4 errors detected in the compilation of "/caffe2/caffe2/core/context_gpu.cu".

Pull Request resolved: #89534
Approved by: https://github.com/malfet

caffe2/core/context_gpu.cu

@@ -561,12 +561,12 @@ struct DefaultCUDAAllocator final : public at::Allocator {
           // some models that are currently running with the thc
           // allocator fit in memory.  We will need to find some
           // way of resolving this problem.
-          cuda::CUDAStreamGuard g(
+          c10::cuda::CUDAStreamGuard g(
             Stream(
               Stream::DEFAULT,
               Device(kCUDA, CaffeCudaGetDevice())
             ));
-          ptr = cuda::CUDACachingAllocator::raw_alloc(nbytes);
+          ptr = c10::cuda::CUDACachingAllocator::raw_alloc(nbytes);
         }
         if (FLAGS_caffe2_gpu_memory_tracking) {
           g_size_map[ptr] = nbytes;
@@ -625,7 +625,7 @@ struct DefaultCUDAAllocator final : public at::Allocator {
         break;
       }
       case CudaMemoryPoolType::THC: {
-        cuda::CUDACachingAllocator::raw_delete(ptr);
+        c10::cuda::CUDACachingAllocator::raw_delete(ptr);
         if (FLAGS_caffe2_gpu_memory_tracking) {
           g_cuda_device_affiliation.erase(g_cuda_device_affiliation.find(ptr));
         }