Move `batch`, `sort_embeddings` into `backends/candle`

[Jeadie]

What does this PR do?

• Change: Moves the functions batch, sort_embeddings backends/candle/tests/ to backends/candle.
• Motivation: Crates consuming text-embeddings-inference as a dependency (and not as a standalone server) can access these methods.

Before submitting

• [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• [x] Did you read the contributor guideline, Pull Request section?
• [ ] Was this discussed/approved via a Github issue or the forum? Please add a link to it if that's the case.
• [ ] Did you make sure to update the documentation with your changes? Here are the documentation guidelines, and here are tips on formatting docstrings.
• [x] Did you write any new necessary tests?

Who can review?

Anyone in the community is free to review the PR once the tests have passed.