add Optional to type hints when default is None

[njzjz]

script:

from glob import glob
import re

regex = re.compile(r"([^,\(\)= ]+):[ ]*((?!Optional)[^,\(\)= ]+)[ ]*=[ ]*None")

for ii in glob("deepmd/**/*.py", recursive=True):
    with open(ii, 'r') as ff:
        content = ff.read()        
    has_optional = "Optional" in content
    has_typing = "from typing" in content
        
    content, nsub = regex.subn(r"\1: Optional[\2] = None", content)
    if nsub > 0:
        if not has_optional:
            if has_typing:
                content = content.replace("from typing import", "from typing import Optional,")
            else:
                content = "from typing import Optional\n" + content
            
        with open(ii, 'w') as ff:
            ff.write(content)