FLUX.1-Kontext-dev batch inference throughput issue

Describe the bug

I am trying to use FLUX.1-Kontext-dev in batch inference, but the inference time for batch is linear to batch size, e.g., time of batch size 2 ~= 2* time of batch size 1. This makes me confused, did I get something wrong or this is the issue with this pipeline?

Reproduction

import torch
from diffusers import FluxKontextPipeline
from diffusers.utils import load_image


# Load the pipeline
pipe = FluxKontextPipeline.from_pretrained(
    "black-forest-labs/FLUX.1-Kontext-dev", 
    torch_dtype=torch.bfloat16
)
pipe.to("cuda")

input_images = [
    load_image("https://huggingface.co/datasets/huggingface/documentation-images/resolve/main/diffusers/cat.png"),
    load_image("https://huggingface.co/datasets/huggingface/documentation-images/resolve/main/diffusers/cat.png")
]

prompts = [
    "Make it look like a watercolor painting",
    "Add dramatic lighting and shadows",
]
with torch.inference_mode():
    images = pipe(
        image=input_images,
        prompt=prompts,
        guidance_scale=2.5,
        num_inference_steps=28,
        generator=[torch.Generator("cuda").manual_seed(42),torch.Generator("cuda").manual_seed(123)]
    ).images

for i, img in enumerate(images):
    img.save(f"output_i2i_batch_{i}.png")

Logs

System Info

• 🤗 Diffusers version: 0.36.0.dev0
• Platform: Linux-6.8.0-63-generic-x86_64-with-glibc2.39
• Running on Google Colab?: No
• Python version: 3.10.18
• PyTorch version (GPU?): 2.8.0+cu128 (True)
• Flax version (CPU?/GPU?/TPU?): not installed (NA)
• Jax version: not installed
• JaxLib version: not installed
• Huggingface_hub version: 0.35.3
• Transformers version: 4.57.0
• Accelerate version: 1.10.1
• PEFT version: not installed
• Bitsandbytes version: not installed
• Safetensors version: 0.6.2
• xFormers version: not installed
• Accelerator: NVIDIA H100 NVL, 95830 MiB
  NVIDIA H100 NVL, 95830 MiB
  NVIDIA H100 NVL, 95830 MiB
  NVIDIA H100 NVL, 95830 MiB
  NVIDIA H100 NVL, 95830 MiB
  NVIDIA H100 NVL, 95830 MiB
  NVIDIA H100 NVL, 95830 MiB
  NVIDIA H100 NVL, 95830 MiB

Who can help?

@yiyixuxu @DN6

Start with the reproduced FluxKontextPipeline call in the issue and measure batch sizes 1 and 2 using the supplied environment and inputs. Trace the inference path to determine whether linear timing is expected or indicates a batching problem; done means the behavior is explained and the issue is corrected or documented.