Enable or disable bf16 support based on availability

[simhallq]

Issue #1090

Added support for auto in bf16 setting of config-files. Now sets bf16 to True/False depending on GPU-support if bf16: auto.

[winglian]

Also if you're feeling ambitious, adding a test for normalize config that it sets bf16 appropriately based on a mock of is_torch_bf16_gpu_available.

[simhallq]

@winglian I've changed logging level and added a unit test.

[simhallq]

Btw the test fails on Mac since device=="mps" sets bf16==False (which is the desired behavior), should I mock the value of choose_device in the test or is that overkill?

[winglian]

Perfect. Thanks! No need to worry about mocking the device for now.

[simhallq]

@winglian I just realized I missed something - validate_config now throws an error at this line if bf16 is unavailable, since we are not explicility checking cfg.bf16 is True but just checking for any truthy value (which includes non-empty string like "auto").

Should I open a new PR with the fix?