[Metrics] Add word error metric metric
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Feel free to merge once the CI is all green ;)
This PR adds the word error rate metric to datasets. 
WER: https://en.wikipedia.org/wiki/Word_error_rate
for speech recognition. WER is the main metric used in ASR. 

`jiwer` seems to be a solid library (see https://github.com/asteroid-team/asteroid/pull/329#discussion_r525158939)
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