Fix the namespace for kCudaPlatformId, so cuda_platform.h is consistent with

cuda_platform_id.h

PiperOrigin-RevId: 231504507

tensorflow/stream_executor/cuda/cuda_platform.h

@@ -32,13 +32,14 @@ limitations under the License.
 #include "tensorflow/stream_executor/trace_listener.h"
 
 namespace stream_executor {
-namespace gpu {
-
+namespace cuda {
 // Opaque and unique identifier for the CUDA platform plugin.
 // This is needed so that plugins can refer to/identify this platform without
 // instantiating a CudaPlatform object.
 extern const Platform::Id kCudaPlatformId;
+}  // namespace cuda
 
+namespace gpu {
 // Cuda-specific platform plugin, registered as a singleton value via module
 // initializer.
 class CudaPlatform : public Platform {