[reland] Add formulas and basic tests (#56083)
Summary:
Reland of https://github.com/pytorch/pytorch/pull/49098
See original issue for details.
The only difference with previous PR is the fix of the _embedding_bag_dense_backward formula to stop declaring a backward formula for an argument that does not exists.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/56083
Reviewed By: samestep
Differential Revision: D27778221
Pulled By: albanD
fbshipit-source-id: 159ef91ca931ef2ccfbc3d1c46c7880c32919dc9
