[Bugfix] fix lora_dtype value type in arg_utils.py

[c3-ali]

FILL IN THE PR DESCRIPTION HERE

FIX #5397

[rkooo567]

QQ: does it not support torch.dtype?

[c3-ali]

@rkooo567 It does! I followed the argparse options of choices=['auto', 'float16', 'bfloat16', 'float32'] and used str but it seems LoraConfig specify it as lora_dtype: Optional[torch.dtype] = None and the implementation supports both str and torch.dtype.

    def verify_with_model_config(self, model_config: ModelConfig):
        if self.lora_dtype in (None, "auto"):
            self.lora_dtype = model_config.dtype
        elif isinstance(self.lora_dtype, str):
            self.lora_dtype = getattr(torch, self.lora_dtype)

So lora_dtype: Optional[Union[str, torch.dtype]] = 'auto' is a precise definition. I'm going to make that change.