Transformer final norm preln
wouldn't this break all the checkpoints trained with pre_ln=True? This pull request **introduces 1 alert** when merging 06d1bb03d4144ea1d95b1656083db015711489c2 into d1b37ed30a70bff00df4778b55b020d8bab2e6fb - [view on LGTM.com](https://lgtm.com/projects/g/NVIDIA/NeMo/rev/pr-72c6d188f1fb006ed4703c0de0401a0abd5eb9e6)

**new alerts:**

* 1 for Unused local variable
Fix/Add final layer norm for pre_ln transformer encoder and decoder