fix(mypy): add type stubs and fix typing issues (#3938)

Adds type stubs and fixes mypy errors for better type coverage.

Changes:
- Added type_checking dependency group with type stubs (torchtune, trl,
etc.)
- Added lm-format-enforcer to pre-commit hook
- Created HFAutoModel Protocol for type-safe HuggingFace model handling
- Added mypy.overrides for untyped libraries (torchtune, fairscale,
etc.)
- Fixed type issues in post-training providers, databricks, and
api_recorder

Note: ~1,200 errors remain in excluded files (see pyproject.toml exclude
list).

---------

Co-authored-by: Claude <noreply@anthropic.com>

src/llama_stack/providers/remote/inference/together/together.py

@@ -8,8 +8,8 @@
 from collections.abc import Iterable
 from typing import Any, cast
 
-from together import AsyncTogether
-from together.constants import BASE_URL
+from together import AsyncTogether  # type: ignore[import-untyped]
+from together.constants import BASE_URL  # type: ignore[import-untyped]
 
 from llama_stack.apis.inference import (
     OpenAIEmbeddingsRequestWithExtraBody,