feat: add flux2 small decoder support

[leejet]

.\bin\Release\sd-cli.exe --diffusion-model  ..\..\ComfyUI\models\diffusion_models\flux-2-klein-4b.safetensors --vae ..\..\ComfyUI\models\vae\full_encoder_small_decoder.safetensors  --llm ..\..\ComfyUI\models\text_encoders\qwen_3_4b.safetensors -p "a lovely cat" --cfg-scale 1.0 --steps 4 -v --offload-to-cpu --diffusion-fa -v

[stduhpf]

Here's a quick test comparing output to the original VAE (incuded TAEF2 and an image generated with VAE but saved as jpeg for reference)

-	Flux2 VAE	JPEG	Small Decoder	TAEF2
image
diff
MSE	0	4.8037	5.1254	16.0590
PSNR	inf dB	41.3151 dB	41.0336 dB	36.0736 dB
SSIM	1	0.9814	0.9791	0.9657
Compute buffer	2562.00 MB	2562.00 MB	2178.00 MB	898.00 MB
decode graph time	2.38s	2.38s	1.73s	0.48s

(using --vae-conv-direct, vulkan backend)