Add experimental BF16 support for Metal

[alvarobartt]

What does this PR do?

This PR adds experimental support for BF16 as a supported dtype on Metal, when the features candle and metal are enabled. As candle natively supports BF16, and some recent models as https://huggingface.co/voyageai/voyage-4-nano (and all the other Voyage AI embedding models) default to BF16; support has been introduced to support such cases.

Warning

Support for BF16 is still experimental on Metal, whilst stable for Intel HPUs under the python feature, as it runs with a Python-based backend. And, work in progress for CUDA which requires bumping candle to latest and updating candle-extensions accordingly.

Additionally, this PR also updates the Rust version to 1.92.0 (latest stable) in rust-toolchain.toml, and removes half as unused.

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 documentation guidelines.
• Did you write any new necessary tests? If applicable, did you include or update the insta snapshots?

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