Fix TYPE_CHECKING not imported
# What does this PR do?

Fixes omitted import of TYPE_CHECKING in xlm_prophetnet model code's __init__.py.

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- [ ] Did you write any new necessary tests?


Actually the whole init needs to be rewritten to be the same as other models, it was somehow missed when we converted all models. Would like to amend your PR in that direction?<|||||>Your rebase has introduced many file changes in the diff that make the PR unreadable. Once you're satisfied with your branch, could you close this PR and open a new one? This should make the diff better.<|||||>Sure thing.