[Timit_asr] Make sure not only the first sample is used 
cc @lhoestq  @vrindaprabhu Failing `run (push)` is unrelated -> merging Thanks for fixing this, it was affecting my runs for https://github.com/huggingface/transformers/pull/10581/ I am seeing this very late! Sorry for the blunder everyone! :(
When playing around with timit I noticed that only the first sample is used for all indices. I corrected this typo so that the dataset is correctly loaded.