load training data once from disk if there is only one set

[wanghan-iapcm]

No description provided.

[codecov-commenter]

Codecov Report

Base: 75.64% // Head: 75.67% // Increases project coverage by +0.02% 🎉

Coverage data is based on head (e144303) compared to base (2abf846).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##            devel    #2264      +/-   ##
==========================================
+ Coverage   75.64%   75.67%   +0.02%     
==========================================
  Files         204      204              
  Lines       20266    20267       +1     
  Branches     1438     1438              
==========================================
+ Hits        15331    15337       +6     
  Misses       4028     4028              
+ Partials      907      902       -5     

Impacted Files	Coverage Δ
deepmd/utils/data.py	93.64% <100.00%> (+0.02%)	⬆️
source/lib/include/ComputeDescriptor.h	77.31% <0.00%> (+0.66%)	⬆️
source/lib/src/fmt_nlist.cc	80.72% <0.00%> (+2.40%)	⬆️

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