add Optional to type hints when default is None

[njzjz]

This PR does the same thing as #2139.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.03 ⚠️

Comparison is base (e37f45c) 74.86% compared to head (b4d060c) 74.84%.

Additional details and impacted files

@@            Coverage Diff             @@
##            devel    #2361      +/-   ##
==========================================
- Coverage   74.86%   74.84%   -0.03%     
==========================================
  Files         223      223              
  Lines       22270    22272       +2     
  Branches     1633     1633              
==========================================
- Hits        16673    16669       -4     
- Misses       4536     4545       +9     
+ Partials     1061     1058       -3     

Impacted Files	Coverage Δ
deepmd/descriptor/loc_frame.py	95.31% <ø> (ø)
deepmd/descriptor/se_a_ebd.py	13.28% <ø> (ø)
deepmd/descriptor/se_a_ef.py	53.55% <ø> (ø)
deepmd/descriptor/se_a_mask.py	85.45% <ø> (ø)
deepmd/descriptor/se_r.py	96.49% <ø> (ø)
deepmd/descriptor/se_t.py	94.87% <ø> (ø)
deepmd/fit/ener.py	89.36% <ø> (ø)
deepmd/infer/deep_eval.py	93.10% <ø> (ø)
deepmd/model/ener.py	95.61% <ø> (ø)
deepmd/model/tensor.py	88.78% <ø> (ø)
... and 12 more

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

☔ View full report at Codecov.
📢 Do you have feedback about the report comment? Let us know in this issue.