Replace Vec256<T>::size with constexpr method (#15406)
Summary:
Stack:
:black_circle: **#15406 Replace Vec256<T>::size with constexpr method** [:yellow_heart:](https://our.intern.facebook.com/intern/diff/D13519902/)
See Note [constexpr static function to avoid odr-usage compiler bug]
for detailed justification.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15406
Differential Revision: D13523774
Pulled By: ezyang
fbshipit-source-id: c0ab44298bb2ef3d68a66d026fc6bc156a909a6b
