Code compatibility with python < 3.9

[brendonlutnick]

It seems that the files "deeplab2/model/layers/moat_blocks.py" and "deeplab2/model/pixel_encoder/moat.py" are using list[int] and List[int] interchangeably in function definitions. From what I can gather - this works in python 3.9 but not in previous versions. I was able to make the code run in python 3.8 by editing the files to use List[int], and Dict[int] and importing List and Dict from typing.