Alias for `digamma` as `psi` to `special` namespace (#59143)
Summary:
See https://github.com/pytorch/pytorch/issues/50345
cc: mruberry kshitij12345
Pull Request resolved: https://github.com/pytorch/pytorch/pull/59143
Reviewed By: jbschlosser
Differential Revision: D28986909
Pulled By: mruberry
fbshipit-source-id: bc8ff0375de968f3662b224689fa0a6b117f9c4e
