bf16: auto support

[winglian]

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🔖 Feature description

rather than having to have the user figure out if their gpu supports bf16, we can have an bf16: auto and then do detection if bf16 is supported and set bf16/fp16 appropriately.

✔️ Solution

see above

❓ Alternatives

No response

📝 Additional Context

No response

Acknowledgements

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[winglian]

Thanks @simhallq. Closed by #1116