[Wav2Vec2] Make sure that gradient checkpointing is only run if needed
Author: patrickvonplaten
Pull request: yes

# What does this PR do?

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Further fixes: https://github.com/huggingface/transformers/issues/14388 - thanks @MarktHart for noticing the degradation.


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Comments:
This PR further reduces the required memory by 20%

cc @anton-l FYI