Updated Gemma configs and Documentation

[pbontrager]

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)

Changelog

The default paths for Gemma 2B don't match the our current default download path. Updated the paths. I also added a link for each model family in the models ref to link to the page where you can request permissions to download a model. This makes it easy to click to the right spot to request access for each model.

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.)

• 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
• run unit tests via pytest tests
• run recipe tests via pytest tests -m integration_test
• manually run any new or modified recipes with sufficient proof of correctness
  • include relevant commands and any other artifacts in this summary (pastes of loss curves, eval results, etc.)

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/torchtune/1190

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As of commit c7f0240 with merge base 8e03661 ():
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[RdoubleA]

one nit: can you point to the instruct model here instead?