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# What does this PR do?

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Model Class attributes `_tied_weights_keys`, `
_keys_to_ignore_on_load_missing` can only be `None` or a List.
`getattr(class_, "_keys_to_ignore_on_load_missing", [])` will return
`None` if `_keys_to_ignore_on_load_missing` is None, and
`discard_names.extend(None)` will trigger an exception, even though
`_tied_weights_keys` exists.

## Who can review?

@OlivierDehaene  @Narsil

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Co-authored-by: Nicolas Patry <patry.nicolas@protonmail.com>