Add missing type hints in checkpointing.py

[RudrenduPaul]

What this PR does

Adds missing type hints to four functions in src/accelerate/checkpointing.py:

• save_accelerator_state: adds the missing -> Path return type (the
  function already returns output_dir, a Path, but the annotation was
  absent even though every parameter was already typed).
• load_accelerator_state: was entirely untyped. Adds input_dir: str,
  models: list, optimizers: list, schedulers: list,
  dataloaders: list, process_index: int, scaler: Optional[GradScaler] = None, load_kwargs: Optional[dict] = None, and a -> dict return
  type. Each of these mirrors the type already established for the
  equivalent parameter in the sibling save_accelerator_state function
  in the same file (e.g. process_index: int and scaler: Optional[GradScaler] = None are copied verbatim), and the -> dict
  return type matches the function's own docstring (Returns: dict) and
  the actual returned value (override_attributes = dict()).
• save_custom_state / load_custom_state: adds path: str (matching
  the output_dir: str / input_dir: str convention used elsewhere in
  the file) and -> None (both functions have no return statement).

Scope note: intentionally left untyped

• obj in save_custom_state / load_custom_state: this is a
  duck-typed parameter (only requires .state_dict() /
  .load_state_dict()), and there's no established precedent for its
  type in this file, so it was left unannotated rather than guess.
• map_location in load_accelerator_state: the docstring says str,
  but the actual runtime value is reassigned to a PartialState().device
  object when "on_device" is passed, so a single accurate type isn't
  obvious from reading the function. Left unannotated rather than
  introduce an inaccurate hint.
• load_model_func_kwargs (**kwargs): left untyped, consistent with
  how **kwargs is handled elsewhere in this typing-cleanup series.

No behaviour change

Type-hint-only change. No logic was modified.

Testing

• ruff check src/accelerate/checkpointing.py — 3 pre-existing findings
  (an unnecessary dict() call, a enumerate() index-lookup nit, and a
  blind except Exception), none on changed lines, none introduced by
  this change (verified against upstream main before/after).
• ruff format --check src/accelerate/checkpointing.py — passes, file
  already formatted.
• python3 -m py_compile on the modified file — passes.

Related PRs

Follows the same pattern as my other open type-hint PRs in this repo:
#4123, #4124, #4125.

Note: Claude Code was used to assist in drafting this change. All
changes were reviewed by the submitter.