Add tests to `sigmoid_backward` and `fmod` (#42289)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/42289
`sigmoid_backward` and `fmod` are not covered by neither in `test/cpp/api` nor in `Aten/test`. Add test functions to cover them
Test Plan:
1. Test locally and check new lines are covred
2. CI
Reviewed By: malfet
Differential Revision: D22804912
fbshipit-source-id: ea50ef0ef3dcf3940ac950d74f6f1cb38d8547a7
