Use PrecisionType enum instead of checking raw values

[awaelchli]

What does this PR do?

Addresses comment
We have an enum that covers several precision types such as 16 bit, "bf16", "mixed", etc.
This PR makes sure we use the enum fields instead of the raw values across the code base.

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Did you have fun?

I made sure I had fun coding 🙃

Part of #1 (it's a lie, this is just here to avoid noisy GitHub bot)

cc @Borda @justusschock @awaelchli @akihironitta @carmocca