Fix __constants__ for some nn modules (#21071)
Summary:
A bunch of modules were missing entries for `__constants__` which was making their `__repr__`s not work. Others had `__constants__` that were not necessary since it was provided by some parent class instead.
Fixes #20978
](https://our.intern.facebook.com/intern/diff/15539518/)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21071
Pulled By: driazati
Differential Revision: D15539518
fbshipit-source-id: 24bdd1ef41ef636eefd5d2bad4ab2d79646ed4f0
