Replace Vec256<T>::size with constexpr method (#15406)

Replace Vec256<T>::size with constexpr method (#15406)

Summary:
Stack:
&nbsp;&nbsp;&nbsp;&nbsp;:black_circle:&nbsp; **#15406 Replace Vec256<T>::size with constexpr method**&nbsp;&nbsp;[: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