ModularPipeline.save_pretrained: change the default overwrite_modular_index to be True

[yiyixuxu]

cc @DN6

Just a proposal, but I think since save_pretrained now saves the loaded components by default, it makes sense for overwrite_modular_index to default to True as well — otherwise the result is not very intuitive: e.g. when you run

  pipe = ModularPipeline.from_pretrained(repo_id)
  pipe.load_components() 
  pipe.save_pretrained(local_dir)

the components are all saved into local_dir, but its modular_model_index.json still points at repo_id — so reloading from the local_dir still fetch everything from hub even though weights are in the local folder

  pipe = ModularPipeline.from_pretrained(local_dir)
  pipe.load_components()

It's especially confusing if you intended to save a self-contained local copy with some custom components

See related issues: #14251, #14640

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