Added mask_time_prob and mask_time_length arguments to wav2vec2 pretraining script
This PR relates to [PR 19997](https://github.com/huggingface/transformers/pull/19997), in which I messed up the PR by forgetting the --force flag when pushing. Hopefully this PR is correctly performed.

@sanchit-gandhi @sgugger @patrickvonplaten 
_The documentation is not available anymore as the PR was closed or merged._<|||||>@sanchit-gandhi 