Winogrande task

[HaokunLiu]

This PR adds Winogrande v1.1 beta task.
Reported dev set acc. for XL training size using roberta-large 0.793
dev set acc. (batch size = 4, dropout and lr as shown below) from our implementation

	dropout=0.2	dropout=0.1
lr=2e-5	0.515	0.517
lr=1e-5	0.513	0.528
lr=5e-6	0.791	0.791
lr=3e-6	0.526	0.800

[sleepinyourhat]

Neat, thanks! @pyeres, mind taking this on?

[pep8speaks]

Hello @HaokunLiu! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file jiant/tasks/tasks.py:

Line 2729:99: W291 trailing whitespace

You can repair most issues by installing black and running: black -l 100 ./*. If you contribute often, have a look at the 'Contributing' section of the README for instructions on doing this automatically.

[pyeres]

Looks good!