Summary

This PR preserves required custom block inputs when generating a Mellon config.

Motivation

While reviewing the Mellon custom block flow, I noticed that MellonPipelineConfig.from_custom_block() always emitted an empty
equired_inputs list, even when a block input was declared with
equired=True. The same code path also printed a debug line for every input.

This makes the generated Mellon schema less accurate for custom blocks and can leak noisy stdout from a library API. This came up while reading through the modular Mellon findings in #13650.

Changes

• track required custom block inputs in rom_custom_block()
• stop printing a debug line for each processed input
• add a regression test covering the generated
  equired_inputs metadata

Testing

• python -m py_compile src/diffusers/modular_pipelines/mellon_node_utils.py tests/modular_pipelines/test_modular_pipelines_custom_blocks.py

Notes

I could not run the pytest target in this local environment because pytest and the repo's development dependencies are not installed here, so I limited verification to a syntax check and a focused regression test addition.