Make checkpoint_sequential work with multiple arguments (#14278)

Make checkpoint_sequential work with multiple arguments (#14278)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/14278

In this commit, we make checkpoint_sequential work for models with multiple tensor inputs. Previously, it only processed the first tensor and ignored the rest.

We introduce a new test in test/test_utils.py that replicates the issue referenced in this [GitHub issue](https://github.com/pytorch/pytorch/issues/11093), and we make sure that the test passes by changing the behavior of checkpoint_sequential to process all input tensors.

Reviewed By: ezyang

Differential Revision: D13144672

fbshipit-source-id: 24f58233a65a0f5b80b89c8d8cbced6f814004f7