piecewise_constant is the only diffusers scheduler that needs a step_rules string, and
neither diffusion trainer passes one (get_scheduler is called with only warmup/training
steps, and there is no config field for it). Accepting it let /diffusion/start pass
normalized(), free the resident GPU workloads, spawn the trainer, and only then crash in
the subprocess (get_piecewise_constant_schedule does step_rules.split(",") on None) -- the
exact evict-then-fail the up-front validation exists to prevent. Reject it now with a clear
400. The remaining six schedulers all run with only warmup/training steps.
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