add a comment to native_functions explaining softmax interfaces (#22651)
Summary:
Address the review comment made by gchanan here:
https://github.com/pytorch/pytorch/pull/22456#discussion_r300715866
Pull Request resolved: https://github.com/pytorch/pytorch/pull/22651
Differential Revision: D16181828
Pulled By: nairbv
fbshipit-source-id: 0d41a9024c2664298c281e198a997be73e7f8499
