What does this PR do?

Summary

This PR fixes a redundant terminal denoising step in Z-Image pipelines.

Previously, Z-Image pipelines mutated the scheduler with scheduler.sigma_min = 0.0.
With FlowMatchEulerDiscreteScheduler, this caused the generated timestep/sigma schedule to include a model-forward step at terminal sigma 0.0, while the scheduler also appended its own terminal 0.0. The final denoising step therefore became a no-op transition from 0.0 -> 0.0.

This change makes Z-Image pipelines compute their default sigma schedule up front and pass it through the existing sigmas argument, instead of mutating scheduler.sigma_min.

Behavior

Old Schedule

New Schedule

Result Check

Generated 1024x1024 images with Tongyi-MAI/Z-Image-Turbo, prompt dance monkey, seed 0.

New 8 steps

Old 9 steps

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@sayakpaul