Add type hints

[pietermarsman]

I use mypy and want to have type hints for retrying to fix this error:

error: Skipping analyzing "efficientnet_pytorch": module is installed, but missing library stubs or py.typed marker  [import]
error: Call to untyped function (unknown) in typed context  [no-untyped-call]

Current non-optimal solution is to ignore by adding the following lines to the mypy.ini config.

[mypy-efficientnet_pytroch.*]
ignore_missing_imports = True