Add WMT20 MLQE 3 shared tasks
Thanks for the comments Quentin!
I integrated them It should be ok now!
Sorry I wasn't attentive enough.
(tests are currently failing, I understand it's from other datasets) merging since the CI is fixed on master
3 tasks for the WMT 20 MLQE shared tasks -> 3 different datasets

(I re-created #1137 because it was too messy).

Note that in L199 `task3.py`, I used `logging.warning` to print some missing data in the train set.