Add Mahalanobis distance metric
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Mahalanobis distance is a very useful metric to measure the distance from one datapoint X to a distribution P. 
In this PR I implement the metric in a simple way with the help of numpy only.

Similar to the [MAUVE implementation](https://github.com/huggingface/datasets/blob/master/metrics/mauve/mauve.py), we can make this metric accept texts as input and encode them with a featurize model, if that is desirable.
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