Fix the ELU formula in the docs (#43764)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/43389.
This PR replaces the old ELU formula from the docs that yields wrong results for negative alphas with the new one that fixes the issue and relies on the cases notation which makes the formula more straightforward.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/43764
Reviewed By: ailzhang
Differential Revision: D23425532
Pulled By: albanD
fbshipit-source-id: d0931996e5667897d926ba4fc7a8cc66e8a66837
