code: def _warn_to_gpu(src_device, dst_device):
      src_id = src_device.device.id
      dst_id = dst_device.device.id
      if src_id != dst_id:
          if _thread_local.flag_gpu_to_gpu is None:
              warnings.warn(
                  """\
  You are trying to transfer a DeviceResident to GPU-{dst} which is already on \
  GPU-{src}.
  `DeviceResident.to_gpu` does nothing if the DeviceResident is already on GPU.
  
  You can use `DeviceResident.to_device()` method to perform inter-GPU transfer.
  """.format(
                      dst=dst_id, src=src_id
                  ),
                  RuntimeWarning,
              )
          else:
              assert isinstance(_thread_local.flag_gpu_to_gpu, bool)
              _thread_local.flag_gpu_to_gpu = True
  
imports: from __future__ import absolute_import
  import abc
  import sys
  import threading
  import typing
  import warnings
  import numpy
  import chainer
  from chainer import backend
  from chainer.backends import _cpu
  from chainer.backends import cuda
  from chainer.backends import intel64
  from chainer import types
  from chainer import utils
  import chainerx
label_desc: Use image node_id 7 for calling the _ToDeviceVisitor obj's underlying member method code with example usage: obj._warn_to_gpu(src_device, dst_device) without return types