Add variable_list support to ExtractVariables struct (#84583)
This is required to unblock https://github.com/pytorch/xla/pull/3843, which lowers the einsum op for pytorch/xla. Because one method input parameter is a TensorList, we need to support TensorLists here so that we can support einsum gradients.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/84583
Approved by: https://github.com/soulitzer
