[Modular] Avoid downloading weights when loading from an existing local path

[DN6]

What does this PR do?

As discussed in here.

This PR:

• Checks if a component is being loaded from a local path
• If the component is a model, it checks if weight files exist in the local path. If it is a scheduler/tokenizer etc, it checks for the relevant configs in the local path.
• If the component is determined to exist locally, we opt to load from there instead of the using the hub repo id.
• If the component does not exist locally e.g. A text encoder pointing to another repo, it gets downloaded from the hub

# assuming you have run something like this beforehand
# hf download <repo-id> --local-dir <my local path>

pipe = ModularPipeline.from_pretrained(<my local path>)
pipe.load_components() # no longer re-downloads components

Fixes # (issue)

Before submitting

• Did you use an AI agent (Claude Code, Codex, Cursor, etc.) to help with this PR? If so:
  • Did you read the Coding with AI agents guide?
  • Did you run the self-review skill on the diff?
  • Did you share the final self-review notes in the PR description or a comment?
• Did you read the contributor guideline?
• Did you read our philosophy doc? (important for complex PRs)
• Was this discussed/approved via a GitHub issue or the forum? Please add a link to it if that's the case.
• Did you make sure to update the documentation with your changes? Here are the
  documentation guidelines, and
  here are tips on formatting docstrings.
• Did you write any new necessary tests?
• Are you the author (or part of the team) of the model/pipeline (only applicable for model/pipeline related PRs)?

Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.