Add MNLI to Full Benchmark

[epavlick]

coordinate with whoever is working on SuperGLUE, we only need to include MNLI once. But NLI will be held-out from model training (whereas the other SuperGLUE tasks will not) so interpreting MNLI results is different from other superglue tasks.

use to test generalization to unseen task; maybe use FLEX?

[PierreColombo]

I can do it :)

[wilsonyhlee]

@PierreColombo I'd love to help contribute to this one if you need any help!

[manandey]

MNLI in PS here