Fix fairseq wav2vec2-xls-r pretrained weights conversion scripts
# What does this PR do?
Fixes:  #19319

This PR fixes some bug on wav2vec2 fairseq weights conversion scripts, that [wav2vec2-xls-r-kind](https://github.com/facebookresearch/fairseq/tree/main/examples/wav2vec/xlsr) weights files fail to be loaded on this line.

https://github.com/huggingface/transformers/blob/bc21aaca789f1a366c05e8b5e111632944886393/src/transformers/models/wav2vec2/convert_wav2vec2_original_pytorch_checkpoint_to_pytorch.py#L249

This can be resolved by specifying fairseq task as `audio_pretraining`, and loading fairseq weights with the task context.

This change follows the way of fairseq library, which loads pretrained model weights via passing `task` argument on cli. 
Conversion of other non-finetuned weights works without any side effects. (tested with [wav2vec2-base](https://dl.fbaipublicfiles.com/fairseq/wav2vec/wav2vec_small.pt), [wav2vec2-conformer](dl.fbaipublicfiles.com/fairseq/conformer/wav2vec2/librilight/LL_relpos_PT_no_FT)) 

Referenced model weights are in the following url: https://github.com/facebookresearch/fairseq/tree/main/examples/wav2vec

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@sanchit-gandhi
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