Export group norm as ATen and add test (#15569)
Summary:
Short term solution, export group norm as an ATen op to unblock users.
Long term will add GroupNorm to onnx.
Add an end to end test for this one.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15569
Differential Revision: D13554293
Pulled By: houseroad
fbshipit-source-id: b4974c9ea2a1b81338ca1e5c6747efe2715d7932
