Add colab notebook for fine-tuning mT5 on XNLI

[mayhewsw]

This is basically copying the t5-trivia notebook from the T5 repo.

I've run this with mT5-small for a small number of steps (5700), and got the following validation accuracies.

INFO:absl:eval/xnli_dev_test.all_langs/accuracy at step 1005700: 65.673
INFO:absl:eval/xnli_dev_test.ar/accuracy at step 1005700: 62.610
INFO:absl:eval/xnli_dev_test.bg/accuracy at step 1005700: 68.956
INFO:absl:eval/xnli_dev_test.de/accuracy at step 1005700: 68.072
INFO:absl:eval/xnli_dev_test.el/accuracy at step 1005700: 67.671
INFO:absl:eval/xnli_dev_test.en/accuracy at step 1005700: 79.960
INFO:absl:eval/xnli_dev_test.es/accuracy at step 1005700: 72.129
INFO:absl:eval/xnli_dev_test.fr/accuracy at step 1005700: 69.157
INFO:absl:eval/xnli_dev_test.hi/accuracy at step 1005700: 61.446
INFO:absl:eval/xnli_dev_test.ru/accuracy at step 1005700: 67.309
INFO:absl:eval/xnli_dev_test.sw/accuracy at step 1005700: 55.422
INFO:absl:eval/xnli_dev_test.th/accuracy at step 1005700: 64.177
INFO:absl:eval/xnli_dev_test.tr/accuracy at step 1005700: 61.767
INFO:absl:eval/xnli_dev_test.ur/accuracy at step 1005700: 57.912
INFO:absl:eval/xnli_dev_test.vi/accuracy at step 1005700: 63.333
INFO:absl:eval/xnli_dev_test.zh/accuracy at step 1005700: 65.181

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[mayhewsw]

@googlebot I signed it!

[lintingxue]

Hey Stephen, thanks for the suggestion, unfortunately we don't accept external PRs for this repo.