[XLA:Python] Fix potential issue in CopyToDevice where source device buffer might get deallocated before transfer completes.

PiperOrigin-RevId: 257216762

tensorflow/compiler/xla/python/local_client.cc

@@ -652,6 +652,10 @@ StatusOr<std::unique_ptr<PyLocalBuffer>> PyLocalBuffer::CopyToDevice(
         output_buffer));
   }
 
+  // We hold on to the `src_device_buffer` until the transfer is finished.
+  src_device.ThenRelease(src_device_to_device_stream,
+                         std::move(src_device_buffer));
+
   // Write new tuple buffers. The destination buffers have different addresses,
   // so we must construct tuple buffers from scratch instead of copying them.
   if (dst_buffer.on_device_shape().IsTuple()) {