Fix QuantState and dict conversions

[cyyever]

Fix the failure of QuantState.as_dict then followed by QuantState.from_dict.
A reproducing example is

import torch

from bitsandbytes.functional import (
    QuantState,
    dequantize_4bit,
    quantize_4bit,
)


a = torch.rand(2, 3)
quantized, quant_state = quantize_4bit(A=a, quant_type="nf4")
quant_dict = quant_state.as_dict()
quant_state = QuantState.from_dict(quant_dict, device=torch.device("cuda"))
quantized = dequantize_4bit(A=quantized, quant_state=quant_state)

Before this PR, it failed with (on bitsandbytes 0.47.0):

  File "/home/cyy/a.py", line 13, in <module>
    quant_state = QuantState.from_dict(quant_dict, device=torch.device("cuda"))
  File "/home/cyy/.local/lib/python3.13/site-packages/bitsandbytes/functional.py", line 469, in from_dict
    raise ValueError(
        f"There should be exactly one `quant_state` item with ending from {cls.valid_qs_type_keys}.\nDetected {qs_key}.",
    )
ValueError: There should be exactly one `quant_state` item with ending from ['bitsandbytes__fp4', 'bitsandbytes__nf4'].
Detected [].

[matthewdouglas]

Hi,
Can you provide more detail on the issue that this fixes? A simple reproducer with example output would be ideal. Thanks!

[cyyever]

@matthewdouglas Added