code: def underlying_variable_ref(t):
    
    while t.op.type in ["Identity", "ReadVariableOp", "Enter"]:
      t = t.op.inputs[0]
  
    op_type = t.op.type
    if "Variable" in op_type or "VarHandle" in op_type:
      return t
    else:
      return None
docs: Find the underlying variable ref.
  
    Traverses through Identity, ReadVariableOp, and Enter ops.
    Stops when op type has Variable or VarHandle in name.
  
    Args:
      t: a Tensor
  
    Returns:
      a Tensor that is a variable ref, or None on error.