Fix modular branch input defaults

[pzarzycki]

Summary

Fixes modular pipeline input default handling so branch-local defaults do not leak across conditional branches.

In ConditionalPipelineBlocks, pipeline-level defaults are now only exposed when every branch declares the same default for a shared input. If defaults differ across branches, or only some branches declare the input, the pipeline-level default stays unset and the selected branch applies its own local default.

This fixes cases where one branch could not distinguish:

• user explicitly passed a value
• a sibling branch's default was filled into PipelineState before branch selection

Fixes #14132.

Repro

Minimal shape of the bug:

• one branch declares num_frames=189
• sibling branch declares num_frames=None and requires that it stay unset when action is provided
• before this change, the merged pipeline-level default filled num_frames=189 into state even on the action branch
• after this change, the action branch receives num_frames=None unless the user explicitly passed it

Implementation

• changed conditional input aggregation in src/diffusers/modular_pipelines/modular_pipeline.py
• preserved pipeline-level defaults only when all branches agree on the same default
• kept selected block local defaults working by not materializing missing None defaults into PipelineState
• added focused regression tests for:
  • branch-local default leak repro
  • default-branch local default preservation
  • hidden branch-local defaults in default_call_parameters
  • preserved shared defaults
  • conflicting non-None defaults not being promoted

Tests

Ran:

./.venv/bin/python -m pytest tests/modular_pipelines/test_conditional_pipeline_blocks.py -q

Result:

26 passed

Additional broader validation surfaced unrelated modular test failures that are not addressed in this PR:

• ErnieImage batch/single parity
• LTX batch/single parity
• Helios auto CPU offload device mismatch

[Copilot]

Pull request overview

Fixes modular pipeline input default handling for ConditionalPipelineBlocks so branch-local defaults don’t get promoted to pipeline-level defaults (and therefore don’t leak into sibling branches before branch selection). This aligns behavior with the intent described in #14132: only defaults that are truly shared across all branches should appear as pipeline-level defaults.

Changes:

• Updated conditional input aggregation to only promote a pipeline-level default when every branch declares the same default for a shared input.
• Adjusted state/input resolution so missing inputs fall back to the selected block’s local default, and None defaults are not materialized into PipelineState before branch selection.
• Added regression tests covering branch-local default leakage, shared default promotion, and conflicting-default non-promotion.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/diffusers/modular_pipelines/modular_pipeline.py	Changes how conditional inputs are combined and how defaults are applied so branch-local defaults don’t leak across branches.
tests/modular_pipelines/test_conditional_pipeline_blocks.py	Adds focused regression tests for branch-local defaults, shared defaults, and conflicting defaults behavior.

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