Addd options to use updated bleurt checkpoints
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Adds options to use newer recommended checkpoint (as of 2021/10/8) bleurt-20 and its distilled versions. 

Updated checkpoints are described in https://github.com/google-research/bleurt/blob/master/checkpoints.md#the-recommended-checkpoint-bleurt-20

This change won't affect the default behavior of metrics/bleurt. It only adds option to load newer checkpoints as

`datasets.load_metric('bleurt', 'bleurt-20')`

`bluert-20` generates scores roughly between 0 and 1, which wasn't the case for the previous checkpoints. 
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