[BE] Tweak Meta copyright headers (#90805)
s/Facebook, Inc./Meta Platforms, Inc/
s/Confidential and proprietary./This source code is licensed under the BSD-style license/
Per https://www.internalfb.com/intern/wiki/Open_Source/Licenses/Straight_BSD/
Also, add linter that prevents adding those in the future
Fixes https://github.com/pytorch/pytorch/issues/90187
Pull Request resolved: https://github.com/pytorch/pytorch/pull/90805
Approved by: https://github.com/zpao
