add the missing sstream header

[njzjz]

Fix #1815.

[codecov-commenter]

Codecov Report

Merging #1817 (9c86838) into devel (c4f0cec) will increase coverage by 0.84%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##            devel    #1817      +/-   ##
==========================================
+ Coverage   76.15%   76.99%   +0.84%     
==========================================
  Files          94      117      +23     
  Lines        7850     9413    +1563     
==========================================
+ Hits         5978     7248    +1270     
- Misses       1872     2165     +293     

Impacted Files	Coverage Δ
deepmd/common.py	79.35% <0.00%> (-3.04%)	⬇️
deepmd/entrypoints/convert.py	15.38% <0.00%> (-2.80%)	⬇️
deepmd/fit/ener.py	89.91% <0.00%> (-2.33%)	⬇️
deepmd/env.py	76.14% <0.00%> (-2.16%)	⬇️
deepmd/fit/polar.py	49.01% <0.00%> (-0.74%)	⬇️
deepmd/utils/argcheck.py	89.52% <0.00%> (-0.74%)	⬇️
deepmd/entrypoints/main.py	86.95% <0.00%> (-0.67%)	⬇️
deepmd/utils/convert.py	14.91% <0.00%> (-0.56%)	⬇️
deepmd/descriptor/se_a_ef.py	54.54% <0.00%> (-0.49%)	⬇️
deepmd/train/trainer.py	79.22% <0.00%> (-0.27%)	⬇️
... and 44 more

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