Add type hinting for public functions.

[xingjian-zhang]

Is your feature request related to a problem? Please describe. Currently, our codebase does not contain type hinting for some public functions such as save_task_node_regression : https://github.com/Graph-Learning-Benchmarks/gli/blob/2302ac01fefeb421306d2d8891ad562102a6b5ac/gli/io/node_task.py#L5-L17 It is highly important for every public functions to have explicit type hinting to ease the maintenance and increase readability.

Describe the solution you'd like Add type hinting to every public functions that could potentially called by a user. It should be straightforward to add type hinting given we already specify argument types in most docstrings, though we may need to consider if the docstrings have any misspecification.

Additional context PEP 484