fix batch norm for empty inputs (#30035)
Summary:
Fix for https://github.com/pytorch/pytorch/issues/29578
Shape check is moved up as much as possible, because backends by and large don't correctly handle empty inputs, so check needs to be done before backend selection. That also automatically takes care of backward, because forward for empty input is automatically differentiable, so no backend-specific backward routines are ever called.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/30035
Test Plan: tests for empty inputs are added.
Differential Revision: D18584427
Pulled By: ngimel
fbshipit-source-id: a42918f50eb1f6995921aafa92879cd42dd5e9e1
