refactor: expose shared recipe builders from components

[HuiyingLi]

Summary

• Move duplicated LLM/VLM builder helpers into component-level modules for checkpoint config, distributed setup, loss construction, optimizer construction, LR scheduler construction, step scheduler construction, and wandb setup.
• Export the new public builder paths from the relevant components/*/__init__.py files.
• Keep recipe-level backwards compatibility by importing the moved helpers into the existing recipe modules.
• Update sibling recipes to consume the new component paths where applicable.
• Update Dion optimizer branch tests to exercise the shared component optimizer builder directly.

Validation

• uv run ruff format ... on touched code files
• uv run ruff check ... on touched code files
• Component builder import smoke check
• Shared build_optimizer smoke check
• python -m py_compile ... on touched Python modules
• uv run pytest tests/unit_tests/optim/test_dion_optimizer_utils.py::TestBuildOptimizerDionBranch
• git diff --check

Earlier focused recipe pytest collection was attempted, but this environment fails before test execution due to existing binary/import issues: missing torchvision::nms and a Transformer Engine undefined symbol.

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