[Wav2Vec2] Fix mask_feature_prob
# What does this PR do?

When using `mask_feature_prob > 0` it's important that the `attention_mask` is not passed as the attention_mask's dimension is `[batch_size, sequence_length]`. Since attention_mask = 1 vectors are padded anyways it doesn't matter whether some of those vectors are zero-ed out or not. Thus we should just never pass `attention_mask` for `mask_feature_prob > 0`.
We however need it for `mask_time_prob` since `mask_time_prob` is used for `Wav2Vec2ForPretraining` where the created outputs will be used as labels.
@anton-l - feel free to merge if the change is OK for you :-)<|||||>Makes sense, thanks for adding the tests too! :+1: 