@dataclass on quant configs breaks __eq__/__repr__/hashing, and to_json_string() raises for most configs #15126

Describe the bug

Every quantization config subclass in src/diffusers/quantizers/quantization_config.py is decorated with @dataclass, but each has a custom __init__ and declares no dataclass fields beyond the inherited quant_method. @dataclass still generates __eq__/__repr__ from the declared fields it sees, so:

• GGUFQuantizationConfig(compute_dtype=torch.float16) == GGUFQuantizationConfig(compute_dtype=torch.bfloat16) is True.
• The configs are unhashable.
• repr() hides every real parameter for GGUF, BFL, Nunchaku, Quanto, ModelOpt, and AutoRound configs.

Separately, to_json_string() calls self.to_diff_dict(), which only BitsAndBytesConfig defines — every other config raises AttributeError when to_json_string() is called (which is the default use_diff=True path).

Reproduction

import torch
from diffusers.quantizers.quantization_config import GGUFQuantizationConfig, QuantoConfig

a = GGUFQuantizationConfig(compute_dtype=torch.float16)
b = GGUFQuantizationConfig(compute_dtype=torch.bfloat16)
print(a == b)       # True - should be False
print(repr(a))       # "GGUFQuantizationConfig(quant_method=<QuantizationMethod.GGUF: 'gguf'>)" - compute_dtype is missing
hash(a)               # TypeError: unhashable type: 'GGUFQuantizationConfig'

QuantoConfig(weights_dtype="int8").to_json_string()
# AttributeError: 'QuantoConfig' object has no attribute 'to_diff_dict'

Logs

Traceback (most recent call last):
  File "<string>", line 1, in <module>
TypeError: unhashable type: 'GGUFQuantizationConfig'

Traceback (most recent call last):
  File ".../diffusers/src/diffusers/quantizers/quantization_config.py", line 144, in to_json_string
    config_dict = self.to_diff_dict()
                  ^^^^^^^^^^^^^^^^^
AttributeError: 'QuantoConfig' object has no attribute 'to_diff_dict'

System Info

• diffusers @ main, commit aaf57d0
• Python 3.12, Linux

Who can help?

(leaving blank — no specific quantization reviewer listed in the template's tag guide)