chore(api): add type annotations to LLM class and module-level functions

[Koushik-Salammagari]

What does this PR do?

Adds missing Python 3.10+ style type annotations to litgpt/api.py. api.py had the most unannotated functions of any non-utils.py file in the codebase (11 functions), and also contained several implicit-Optional signatures that mypy flags as errors.

Changes

Location	Change
LLM.__init__	preprocessor: "Preprocessor | None" = None
LLM.__init__	prompt_style: PromptStyle | None = None
LLM.__init__	devices: int | list[int] | None = None
LLM.__init__	config: Config | None = None
LLM.__init__	checkpoint_dir: Path | None = None
LLM.__init__	fabric: L.Fabric | None = None
LLM.tokenizer property	-> Tokenizer
LLM.state_dict	destination: dict | None, prefix: str, keep_vars: bool, -> dict
LLM.load_state_dict	state_dict: dict, strict: bool, -> Any
LLM.benchmark	num_iterations: int, **kwargs: Any, -> tuple[str, dict]
calculate_number_of_devices	devices: int | list[int], -> int
benchmark_dict_to_markdown_table	data: dict, -> str

The preprocessor parameter uses a forward reference string ("Preprocessor | None") because Preprocessor is defined later in the same file.

No new imports needed — Any was already imported from typing.

Before submitting

• Was this discussed/approved via a GitHub issue? Follows the type-annotation pattern from chore(ruff): bump target-version to py310 and update type annotations #2221 and chore(utils): add type annotations to public functions in utils.py #2237.
• Did you make sure to update the docs? N/A — annotation-only change.
• Did you write any new unit tests? N/A — no behavioural change.