fixed a comment string and added a preprocessor branch for ROCM

Signed-off-by: weihanmines <wei.han3@amd.com>

horovod/tensorflow/xla_mpi_ops.cc

@@ -506,7 +506,7 @@ XLAPersistentBuffer::XLAPersistentBuffer(int device, int64_t size)
 #elif HAVE_ROCM
   HVD_GPU_CHECK(hipGetDevice(&restore_device));
   HVD_GPU_CHECK(hipSetDevice(device));
-  // Simply call cudaMalloc for persistent buffer.
+  // Simply call hipMalloc for persistent buffer.
   HVD_GPU_CHECK(hipMalloc((void**)&buffer_, size));
   HVD_GPU_CHECK(hipSetDevice(restore_device));
 #endif
@@ -597,8 +597,13 @@ void CallbackHVDAllreduceDone(gpuStream_t stream, void** /*buffers*/,
   VLOG(2) << "hvd-allreduce-done - End";
 }
 
+#if HAVE_CUDA
 XLA_REGISTER_CUSTOM_CALL_TARGET(CallbackHVDAllreduce, "CUDA");
 XLA_REGISTER_CUSTOM_CALL_TARGET(CallbackHVDAllreduceDone, "CUDA");
+#elif HAVE_ROCM
+XLA_REGISTER_CUSTOM_CALL_TARGET(CallbackHVDAllreduce, "ROCM");
+XLA_REGISTER_CUSTOM_CALL_TARGET(CallbackHVDAllreduceDone, "ROCM");
+#endif
 
 } // namespace
 } // namespace tensorflow