Add missing type hints for PyTorch Longformer models
# What does this PR do?

- Adds missing type hints to the PyTorch Longformer model classes.
    
    - Issue: #16059
    
## Tests / Checks
- I ran `make fixup` before my final commit. 
- `RUN_SLOW=1 py.test -vv tests/longformer/test_modeling_longformer.py` ran without any failures.

_This PR is part of the Great Code Cleanup 2022!_

@Rocketknight1
_The documentation is not available anymore as the PR was closed or merged._