change return dicitonary for DataCollatorForNextSentencePrediction from masked_lm_labels to labels
Author: gmihaila
Pull request: yes

# What does this PR do?

The `masked_lm_labels` argument from  *DataCollatorForNextSentencePrediction*  is deprecated and will be removed in a future version, use `labels` instead. I changed the dictionary key from `masked_lm_labels ` to `labels`.

It will avoid any future errors when `masked_lm_labels` won't be used anymore.

Not a lot of people use the  *DataCollatorForNextSentencePrediction* and I think this will get overlooked in the future if not fixed.

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It fixes warning that appear when using `trainer`

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