Transfer promptsource.seqio_tasks to https://github.com/bigscience-workshop/t-zero

[VictorSanh]

Separate promptsource from the tooling specific to the T0 paper

[VictorSanh]

also change the mixture name to something self-explanatory....

[thomasw21]

Unrelated to the move exactly, but I think we should mention somewhere that running with HF_DATASETS_OFFLINE=1 saves a lot of time when importing those tasks:

> DATASETS_VERBOSITY=error time -p python3 -c "from promptsource import seqio_tasks"
real 199.22
user 36.44
sys 14.02
> HF_DATASETS_OFFLINE=1 DATASETS_VERBOSITY=error time -p python3 -c "from promptsource import seqio_tasks"
real 8.17
user 10.51
sys 12.72

This holds true even if one doesn't actually use the datasets and has already cache the tasks.