[prophetnet] wrong import
```
python -c "from apex.normalization import FusedProphetNetLayerNorm"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
ImportError: cannot import name 'FusedProphetNetLayerNorm' from 'apex.normalization' (/home/stas/anaconda3/envs/main-38/lib/python3.8/site-packages/apex/normalization/__init__.py)
```
It looks like this code has never been tested, so it silently fails inside try/except.

Discovered this by accident in https://github.com/huggingface/transformers/issues/9338#issuecomment-752217708

@patrickvonplaten, @LysandreJik 

note, prophetnet is missing from .github/PULL_REQUEST_TEMPLATE.md, .github/ISSUE_TEMPLATE/bug-report.md