Add custom stopping criteria to ICL generate tasks

[bmosaicml]

What does this PR do?

Modeled after EleutherAI implementation of generate tasks, we enable custom stopping criteria to be implemented.

This relies on five separate changes:

1. Implement a multi token stopping criteria modeled after this
2. Modify QA tasks' collate fn to add stopping criteria to generate kwargs as well as pass them along in the batch as raw strings
3. Modify the QA metric to post-hoc apply the stopping criteria to any outputs it wasn't applied to. This is necessary because stopping criteria don't stop until all outputs in a device batch reach the stopping criteria, so by definition all but one output per batch will need to have it manually applied post-hoc.
4. Modify the split_batch fn to properly handle list values that need to be broadcast (the string literal stopping criteria)
5. Make normalization optional, since GSM8K doesn't use the standard normalization.

What issue(s) does this change relate to?

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[abhi-mosaic]

Couple small questions, logic seems right to me. Would love to see some unit tests.

[bmosaicml]

Couple small questions, logic seems right to me. Would love to see some unit tests.

I'll try and mock the __call__ function somehow in a unit test!

[maxisawesome]

LGTM. Importing a function only to check if it exists is a little funny to me, but I understand the reasoning based on past discussions and think it's a fine solution!