[Wav2Vec2] Padded vectors should not allowed to be sampled
Author: patrickvonplaten
Pull request: yes

# What does this PR do?

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Correct negative sampling for Wav2Vec2ForPreTraining. Previously padded feature vectors could be sampled which would give the model bad signals during pretraining. This PR makes sure that a padded feature vector cannot be sampled as a "negative" vector.


## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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Models:

- albert, bert, xlm: @LysandreJik
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Library:

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Comments:
PR is tested on Wav2Vec2 PreTraining in Flax