[PyTorch] Add test_modules test for TransformerEncoderLayer fast path (#78268)
Extend the existing TransformerEncoderLayer test to cover the fast path.
Differential Revision: [D36564009](https://our.internmc.facebook.com/intern/diff/D36564009/)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/78268
Approved by: https://github.com/zrphercule
