Explicitly support GritLM

[janEbert]

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Why are these changes needed?

Fix GritLM:

• Chat template is now correct.
• Embedding now works correctly, values match the GritLM README example exactly.
• In order to make it easy on users, the general embedding tokens are prepended automatically if no instruction is given. So it's impossible to embed without the embedding instruction.

Related issue number (if applicable)

Ref https://github.com/lm-sys/FastChat/issues/3181, which mentions support for embedding models.

Checks

• [X] I've run format.sh to lint the changes in this PR.
• [ ] I've included any doc changes needed.
• [ ] I've made sure the relevant tests are passing (if applicable).