[Wav2Vec2] Fix normalization for non-padded tensors
# What does this PR do?

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This PR fixes a problem with normalization when the input is a list of different length that is not numpified - see: https://github.com/huggingface/transformers/issues/13504

Just noticed that this bug is pretty severe actually as it affects all large-Wav2Vec2 fine-tuning  :-/.
It was introduced by me in this PR: https://github.com/huggingface/transformers/pull/12804/files - I should have written more and better tests for this. 

=> This means that from transformers 4.9.0 to until this PR is merged the normalization for all large Wav2Vec2 models was way off when fine-tuning the model.
 
@LysandreJik - do you think it might be possible to do a patched release for this? 
