allow to set mxied_precision in accelerate config when using a given deepspeed config

[XiaobingSuper]

What does this PR do?

Fixes #3360 (comment).

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[XiaobingSuper]

The ds config doesn't have such an attribute, and it only works when calling(https://github.com/huggingface/accelerate/blob/main/src/accelerate/accelerator.py#L351)

accelerate/src/accelerate/utils/dataclasses.py

Lines 1256 to 1284 in 81d8a03

	def set_mixed_precision(self, mixed_precision):
	ds_config = self.deepspeed_config
	kwargs = {
	"fp16.enabled": mixed_precision == "fp16",
	# When training in fp8, we still rely on bf16 autocast for the core mixed precision
	"bf16.enabled": mixed_precision in ("bf16", "fp8"),
	}
	if mixed_precision == "fp16":
	if "fp16" not in ds_config:
	ds_config["fp16"] = {"enabled": True, "auto_cast": True}
	elif mixed_precision in ("bf16", "fp8"):
	if "bf16" not in ds_config:
	ds_config["bf16"] = {"enabled": True}
	if mixed_precision == "fp8" and self.enable_msamp:
	if "msamp" not in ds_config:
	ds_config["msamp"] = {"enabled": True, "opt_level": self.msamp_opt_level}
	if mixed_precision != "no":
	diff_dtype = "bf16" if mixed_precision == "fp16" else "fp16"
	if str(ds_config.get(diff_dtype, {}).get("enabled", "False")).lower() == "true":
	raise ValueError(
	f"`--mixed_precision` arg cannot be set to `{mixed_precision}` when `{diff_dtype}` is set in the DeepSpeed config file."
	)
	for dtype in ["fp16", "bf16"]:
	if dtype not in ds_config:
	ds_config[dtype] = {"enabled": False}
	self.fill_match("fp16.enabled", must_match=False, **kwargs)
	self.fill_match("bf16.enabled", must_match=False, **kwargs)

, for the case which the mixed_precision flag is not aligned with the deepspeed config, there will raise error:

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