Minor: add static_assert to Pickler buffering. (#28114)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/28114
This is followup on the pickler buffering change.
ghstack-source-id: 92019521
Test Plan: This just adds an static assert, hence if it builds, we're good.
Differential Revision: D17955006
fbshipit-source-id: d7fd69935d23f39db18029703f63c8f18d23047a
