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).
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• Did you make sure to update the documentation with your changes? Here are the
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  here are tips on formatting docstrings.
• 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.