Add logic for shifting labels before computing metrics

[alextrott16]

What does this PR do?

Adds a shift_labels init argument to HuggingFaceModel class. This instructs the model whether to shift labels by one token before computing metrics, which mimics the way HF Causal LM classes handle labels when computing loss. This fixes the current implementation, which never does this shifting and produces incorrect metric results for Causal LMs.

If shift_labels is not specified, HuggingFaceModel will try to infer the correct behavior based on whether the model is an instance of a registered HF Causal LM class (or a subclass of one).

What issue(s) does this change relate to?

https://mosaicml.atlassian.net/browse/CO-1691

Before submitting

• Have you read the contributor guidelines?
• Is this change a documentation change or typo fix? If so, skip the rest of this checklist.
• Was this change discussed/approved in a GitHub issue first? It is much more likely to be merged if so.
• Did you update any related docs and document your change?
• Did you update any related tests and add any new tests related to your change? (see testing)
• Did you run the tests locally to make sure they pass?
• Did you run pre-commit on your change? (see the pre-commit section of prerequisites)

[dakinggg]

Can you add a test please? Otherwise LGTM. Will approve after test

[dakinggg]

LGTM, left one comment to cleanup the fixture usage a bit (sorry, i pointed you to the wrong place for the usage you wanted)