code: def underlying_variable(t):
    
    t = underlying_variable_ref(t)
    assert t is not None
    # make sure that the graph has a variable index and that it is up-to-date
    if not hasattr(tf.get_default_graph(), "var_index"):
      tf.get_default_graph().var_index = {}
    var_index = tf.get_default_graph().var_index
    for v in tf.global_variables()[len(var_index):]:
      var_index[v.name] = v
    return var_index[t.name]
docs: Find the underlying tf.Variable object.
  
    Args:
      t: a Tensor
  
    Returns:
      tf.Variable.