fix config bug for 'mixed_precision' from 'yaml.safe_load()' (#1386)
* fix config bug for 'mixed_precision' from 'yaml.safe_load()'
* Update src/accelerate/commands/config/config_args.py
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>
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Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>