Update typing in generation_logits_process.py (#12901)

While `Iterable[Iterable[int]]` is a nicer annotation (it's covariant!), the defensive statements parsing out `bad_words_ids` in `__init__(...)` force the caller to pass in `List[List[int]]`. I've changed the annotation to make that clear.

src/transformers/generation_logits_process.py

@@ -354,7 +354,7 @@ class NoBadWordsLogitsProcessor(LogitsProcessor):
             The id of the `end-of-sequence` token.
     """
 
-    def __init__(self, bad_words_ids: Iterable[Iterable[int]], eos_token_id: int):
+    def __init__(self, bad_words_ids: List[List[int]], eos_token_id: int):
 
         if not isinstance(bad_words_ids, List) or len(bad_words_ids) == 0:
             raise ValueError(f"`bad_words_ids` has to be a non-emtpy list, but is {bad_words_ids}.")