Consistent usage of annotation syntax

[f-schnabel]

Is your feature request related to a problem? Please describe.
There are already some files using the new syntax for annotations.
https://github.com/Project-MONAI/MONAI/search?q=from+__future__+import+annotations
The old type aliases from the typing module are deprecated starting with python 3.9,
so to be future-prove it makes sense to migrate to the new style.

Describe the solution you'd like
Use from __future__ import annotations in all source files using annotations, so we have consistent usage (and not only in ~5 files).
The new annotations are also more readable e.g. Optional[Union[List[str], str]] vs. list[str] | str | None.

Secondly, this issue includes changing from typing import Callable, Sequence, ... to from collections.abc import Callable, Sequence, ... since the ones in the typing module are deprecated. They are interchangable even for isinstance checks (see also this stackoverflow thread).

Additional context

• PEP 585
• Documentation to the typing module
• Old discussion about this topic: 4148 Enhance engine iteration logic and the typehints #4150 (comment)

[bhashemian]

Hi @Shadow-Devil, thanks for initiating this improvement.
I agree that updating the typing is a reasonable task since many of them are already deprecated although there is not a defined plan for when they are being removed (maybe in few years).

[f-schnabel]

From the typing module documentation:

The deprecated types will be removed from the typing module in the first Python version released 5 years after the release of Python 3.9.0. See details in PEP 585—Type Hinting Generics In Standard Collections.

So it's not urgent, but I think the new syntax is still more readable and it would be better to have a consistent code style IMO.

[bhashemian]

Exactly, 3.9.0 was released in 2020 so they should be removed in 2025 but no release has officially defined to remove them.