Add type hints to TFPegasusModel

# What does this PR do?

This adds type annotations to `TFPegasusModel` and `TFPegasusForConditionalGeneration` as part of #16059


## Who can review?

@Rocketknight1

Thanks :)

_The documentation is not available anymore as the PR was closed or merged._<|||||>Looks perfect now. Thank you!