Add polygamma where n >= 2 (#42499)
Summary:
https://github.com/pytorch/pytorch/issues/40980
I have a few questions during implementing Polygamma function...
so, I made PR prior to complete it.
1. some code blocks brought from cephes library(and I did too)
```
/*
* The following function comes with the following copyright notice.
* It has been released under the BSD license.
*
* Cephes Math Library Release 2.8: June, 2000
* Copyright 1984, 1987, 1992, 2000 by Stephen L. Moshier
*/
```
is it okay for me to use cephes code with this same copyright notice(already in the Pytorch codebases)
2. There is no linting in internal Aten library. (as far as I know, I read https://github.com/pytorch/pytorch/blob/master/CONTRIBUTING.md)
How do I'm sure my code will follow appropriate guidelines of this library..?
3. Actually, there's a digamma, trigamma function already
digamma is needed, however, trigamma function becomes redundant if polygamma function is added.
it is okay for trigamma to be there or should be removed?
btw, CPU version works fine with 3-rd order polygamma(it's what we need to play with variational inference with beta/gamma distribution) now and I'm going to finish GPU version soon.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/42499
Reviewed By: gchanan
Differential Revision: D23110016
Pulled By: albanD
fbshipit-source-id: 246f4c2b755a99d9e18a15fcd1a24e3df5e0b53e
