What does this PR do?

It enables detection of FLUX.2 Klein models when loading transformers from single file, like in the following example:

import torch
from diffusers import Flux2KleinPipeline, Flux2Transformer2DModel, GGUFQuantizationConfig
dtype = torch.bfloat16

ckpt_path = "https://huggingface.co/unsloth/FLUX.2-klein-4B-GGUF/blob/main/flux-2-klein-4b-Q2_K.gguf"
transformer = Flux2Transformer2DModel.from_single_file(
    ckpt_path,
    quantization_config=GGUFQuantizationConfig(compute_dtype=dtype),
    torch_dtype=dtype,
)
pipe2 = Flux2KleinPipeline.from_pretrained(
    "black-forest-labs/FLUX.2-klein-4B",
    transformer=transformer,
    torch_dtype=dtype,
)
pipe2.enable_model_cpu_offload()

Before this PR, Klein models were detected as flux-2-dev, leading to wrong expected tensor sizes.

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Who can review?

@sayakpaul @yiyixuxu @DN6