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.

def underlying_variable_ref(t):
  """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.
  """
  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