[test] split onnx export test out of test_jit (#40143)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/40143
as titled, to reduce size of test_jit
Test Plan: Imported from OSS
Reviewed By: ailzhang
Differential Revision: D22085036
Pulled By: wanchaol
fbshipit-source-id: 424f189fd3849c111d06ebe2e341da50d98fe0ec
