code: def get_default_mesh() -> Optional[layout_lib.Mesh]:
      """Return the default mesh under the current dtensor device context.
  
      In the case that dtensor device system is not initialized, this function
      will return None.
  
      Returns:
        The current default mesh for the dtensor device context.
      """
      if _dtensor_singleton is None:
          return None
      else:
          return _dtensor_singleton._current_default_mesh
  
imports: import contextlib
  import threading
  from typing import Any, Callable, Optional, Sequence
  from tensorflow.dtensor.python import dtensor_device
  from tensorflow.dtensor.python import gen_dtensor_ops
  from tensorflow.dtensor.python import layout
  from tensorflow.python.eager import context
  from tensorflow.python.framework import ops
  from tensorflow.python.framework import tensor
  from tensorflow.python.util import deprecation
  from tensorflow.python.util.tf_export import tf_export
label_desc: Use image node_id 4 for calling a global function with example usage: get_default_mesh() and returns: None, _dtensor_singleton