[numpy] `torch.digamma` : promote integer inputs to float (#48302)
Summary:
**BC-breaking Note:**
This PR updates PyTorch's digamma function to be consistent with SciPy's special.digamma function. This changes the result of the digamma function on the nonpositive integers, where the gamma function is not defined. Since the gamma function is undefined at these points, the (typical) derivative of the logarithm of the gamma function is also undefined at these points, and for negative integers this PR updates digamma to return NaN. For zero, however, it returns -inf to be consistent with SciPy.
Interestingly, SciPy made a similar change, which was noticed by at least one user: https://github.com/scipy/scipy/issues/9663#issue-396587679.
SciPy's returning of negative infinity at zero is intentional:
https://github.com/scipy/scipy/blob/59347ae8b86bcc92c339efe213128f64ab6df98c/scipy/special/cephes/psi.c#L163
This change is consistent with the C++ standard for the gamma function:
https://en.cppreference.com/w/cpp/numeric/math/tgamma
**PR Summary:**
Reference https://github.com/pytorch/pytorch/issues/42515
Pull Request resolved: https://github.com/pytorch/pytorch/pull/48302
Reviewed By: ngimel
Differential Revision: D25664087
Pulled By: mruberry
fbshipit-source-id: 1168e81e218bf9fe5b849db0e07e7b22e590cf73
