`diffusers-cli custom_blocks` only recognized classes whose base was spelled
exactly `ModularPipelineBlocks`, so a `SequentialPipelineBlocks` or
`AutoPipelineBlocks` subclass (the usual thing to publish) was reported as
"could not be retrieved". Accept the composite subclasses as well, and match
on the class name so dotted bases like
`diffusers.modular_pipelines.SequentialPipelineBlocks` work too.
Co-authored-by: Claude Fable 5.1 <noreply@anthropic.com>