[FIX BUG] type of args in train_instruct_pix2pix_sdxl.py

[Aitical]

What does this PR do?

Fixes type error in default config of args, in example/instruct_pix2pix/train_instruct_pix2pix_sdxl.py

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[putuoka]

how about like this?

from enum import Enum

class VaePrecision(Enum):
  FP32 = "fp32"
  FP16 = "fp16" 
  BF16 = "bf16"

# ...

parser.add_argument(
  "--vae_precision",
  type=VaePrecision,
  choices=list(VaePrecision),
  default=VaePrecision.FP32
)

[sayakpaul]

Thank you!