fix: FSDP initialization for set-valued wrap class names

[Wangxiaoxiaoa]

Description

Normalize FSDP transformer wrap class names before using them during
initialization.

In some cases, these names may be provided as a set rather than a list or
tuple. The previous implementation assumed the value was indexable and could
fail with:

TypeError: 'set' object is not subscriptable

This change normalizes the value before validation and wrapping logic.

Related Issue

Fixes #1186

Type of Change

• 🐛 Bug fix
• ✨ New feature
• 💥 Breaking change
• 📝 Documentation update
• ♻️ Refactoring
• ⚡ Performance improvement
• ✅ Test coverage improvement

Checklist

• I have read the Contributing Guide
• Pre-commit hooks pass (pre-commit run --all-files)
• Relevant tests pass; new tests added for new functionality
• Documentation updated (if applicable; built with ./docs/build_all.sh)
• Branch is up to date with main
• Self-reviewed via /review-pr command
• This PR was created by a coding agent via /create-pr
• This PR is a breaking change

Breaking Change Details (if applicable):

N/A

Additional Context

This is a minimal compatibility fix. It does not change behavior for existing
str or list inputs.

[gemini-code-assist]

Code Review

This pull request introduces a _normalize_wrap_class_names helper function within apply_fsdp2 to standardize the handling of transformer class names, ensuring they are consistently treated as lists regardless of whether the input is a string, None, or an iterable. The review feedback suggests an optimization for this helper function to avoid creating a shallow copy when the input is already a list, thereby reducing unnecessary allocations.