[special] add zeta (#59623)
Summary:
Reference https://github.com/pytorch/pytorch/issues/50345
`zeta` was already present in the codebase to support computation of `polygamma`.
However, `zeta` only had `double(double, double)` signature **for CPU** before the PR (which meant that computation `polygamma` were always upcasted to `double` for zeta part).
With this PR, float computations will take place in float and double in double.
Have also refactored the code and moved the duplicate code from `Math.cuh` to `Math.h`
**Note**: For scipy, q is optional, and if it is `None`, it defaults `1` which corresponds to Reimann-Zeta. However, for `torch.specia.zeta`, I made it mandatory cause for me it feels odd without `q` this is Reimann-Zeta and with `q` it is the general Hurwitz Zeta. I think sticking to just general made more sense as passing `1` for q sounds trivial.
Verify:
* [x] Docs https://14234587-65600975-gh.circle-artifacts.com/0/docs/special.html#torch.special.zeta
Pull Request resolved: https://github.com/pytorch/pytorch/pull/59623
Reviewed By: ngimel
Differential Revision: D29348269
Pulled By: mruberry
fbshipit-source-id: a3f9ebe1f7724dbe66de2b391afb9da1cfc3e4bb
