Wan I2V expand_timesteps: why hard-clamp only first frame (no last-frame clamp)?

[emirks]

I have a question on expand_timesteps option within WanImageToVideoPipeline. I see that the mechanism hard-clamps only the first frame. For FLF (first+last) conditioning, is there a reason we can’t (or shouldn’t) apply the same mechanism to both endpoints?

The related code is in:

• diffusers/src/diffusers/pipelines/wan/pipeline_wan_i2v.py

In prepare_latents, when expand_timesteps is enabled, video_condition ignores last_image and first_frame_mask is defined only for frame 0:

# prepare_latents
if self.config.expand_timesteps:
    video_condition = image

...
if self.config.expand_timesteps:
    first_frame_mask = torch.ones(1, 1, num_latent_frames, latent_height, latent_width, ...)
    first_frame_mask[:, :, 0] = 0
    return latents, latent_condition, first_frame_mask

In the denoising loop, the clamp/mix uses only first_frame_mask:

if self.config.expand_timesteps:
    latent_model_input = (1 - first_frame_mask) * condition + first_frame_mask * latents
    temp_ts = (first_frame_mask[0][0][:, ::2, ::2] * t).flatten()

Question
Is it a limitation of the Wan2.2 I2V checkpoint training (only first-frame conditioning)?
Or a design choice because per-token timestep masking doesn’t extend cleanly to a last-frame constraint?

I see that the corresponding part was written by @yiyixuxu, thanks!