torch.cat and torch.stack batching rules (#43798)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/43798
These are relatively straightforward.
Test Plan: - `pytest test/test_vmap.py -v`
Reviewed By: ezyang
Differential Revision: D23405000
Pulled By: zou3519
fbshipit-source-id: 65c78da3dee43652636bdb0a65b636fca69e765d
