[wav2vec2] fix --gradient_checkpointing
This PR fixes `--gradient_checkpointing` in wav2vec2 examples.

@sgugger 
it was failing deepspeed tests for wav2vec2 - which I still run manually to check that Deepspeed didn't break something. wav2vec2 is a complex model with several manual fixes to make DS work.

but I hear you about the pinned version! This makes sense. I was running those with master, so should probably add a skip rule to match the pinned version. Which would make it much more difficult for me to run the tests. <|||||>@patrickvonplaten, do we merge this or close the PR? Thank you!