This fixes an edge case in the FlowUniPCMultistepScheduler where using high sampling step counts (> 50) would cause an assertion error in the last step. The issue was that with lower_order_final=True, the order calculation could become 0 when step_index equals len(timesteps), causing 'assert self.this_order > 0' to fail.
The fix ensures this_order is always at least 1, maintaining stability while allowing higher quality generation with increased step counts.
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Co-Authored-By: Claude <noreply@anthropic.com>
