code: def merge_device(spec):
      """Returns a device function that merges devices specifications.
  
      This can be used to merge partial specifications of devices. The
      innermost setting for a device field takes precedence. For example:
  
        with tf.device(merge_device("/device:GPU:0"))
          # Nodes created here have device "/device:GPU:0"
          with tf.device(merge_device("/job:worker")):
            # Nodes created here have device "/job:worker/device:GPU:0"
            with tf.device(merge_device("/device:CPU:0")):
              # Nodes created here have device "/job:worker/device:CPU:0"
              with tf.device(merge_device("/job:ps")):
                # Nodes created here have device "/job:ps/device:CPU:0"
  
      Args:
        spec: A `DeviceSpec` or a device spec string (partially) describing the
          device that should be used for all nodes created in the scope of
          the returned device function's with block.
  
      Returns:
        A MergeDevice object with the above-described behavior.
  
      Raises:
        ValueError: if the spec was not valid.
      """
  
      if isinstance(spec, MergeDevice):
          return spec
  
      merger = _cached_mergers.get(spec)
      if merger:
          return merger
      merger = MergeDevice(spec)
      # No locking needed, since updates are stateless.
      _cached_mergers[spec] = merger
      return merger
  
file_path: tensorflow/tensorflow/python/framework/device.py
imports: from tensorflow.python import tf2
  from tensorflow.python.framework import device_spec
label_desc: Use image node_id 4 for calling a global function with example usage: merge_device(spec) and returns: merger, spec, merger