[WHISPER] fix tests after updating the max_length
# What does this PR do?
Fixes the `max_length` used in the generate function in whisper. 
The default `max_length` argument was changed in the `config.json` as it is more convenient for people who either don't know that the argument exist/don't know the value to set. 
_The documentation is not available anymore as the PR was closed or merged._<|||||>Thanks @amyeroberts for running the tests locally, both TF and PT are all green. <|||||>@ArthurZucker It would be nice to explain a bit the reason for the PR, not just what it did 🙏 . It will help others to understand , and it also helps to track (if we need to check this PR for some reason in the future)<|||||>@sgugger 

This is the Hub PR @ArthurZucker mentioned (change `max_length` to 448 in order to fix 2 whisper pipeline tests)
https://huggingface.co/openai/whisper-large/commit/baca495426386f789702e7f10edccf761c5f5592

After that change, this PR is required to pass some TF Whisper tests.

