Undo commenting out NF4/bitsandbytes tests

[SalmanMohammadi]

Context

What is the purpose of this PR? Is it to

• add a new feature
• fix a bug
• update tests and/or documentation
• other (please add here)

My bad. I must've forgot to un-comment this when I was testing locally on MPS, and it landed in #1005. It was just such an exciting time.

Changelog

Un-comment the test file. I've also added a check to skip if bnb isn't installed.

Test plan

Please make sure to do each of the following if applicable to your PR. (If you're not sure about any one of these just ask and we will happily help. We also have a contributing page for some guidance on contributing.)

• run pre-commit hooks and linters (make sure you've first installed via pre-commit install)
• add unit tests for any new functionality
• update docstrings for any new or updated methods or classes
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• run recipe tests via pytest tests -m integration_test
• manually run any new or modified recipes with sufficient proof of correctness
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UX

If your function changed a public API, please add a dummy example of what the user experience will look like when calling it.
Example of docstring:

torchtune/torchtune/modules/vision_transformer.py

Line 285 in 6a7951f

Examples:

Example in our docs: https://pytorch.org/torchtune/main/tutorials/qat_finetune.html#applying-qat-to-llama3-models

• I did not change any public API;
• I have added an example to docs or docstrings;

[pytorch-bot]

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As of commit 398bdc5 with merge base 8bb3a6f ():
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[SalmanMohammadi]

I've also added a check to skip if bnb isn't installed.

This is mostly because it was annoying me but I realise it may not be happening for many people, so don't mind removing it.