feat: calculate the real error in dp model-devi

[njzjz]

This PR adds a new argument dp model-devi --real_error. With that, the real error given in the DP-GEN paper is calculated.

One can use this feature for dpgen simplify if the labeled data exists.

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (1db2195) 78.71% compared to head (27b55ea) 78.72%.

Additional details and impacted files

@@           Coverage Diff           @@
##            devel    #2757   +/-   ##
=======================================
  Coverage   78.71%   78.72%           
=======================================
  Files         241      241           
  Lines       25589    25608   +19     
  Branches     1517     1517           
=======================================
+ Hits        20143    20160   +17     
  Misses       5056     5056           
- Partials      390      392    +2     

Files Changed	Coverage Δ
deepmd/infer/model_devi.py	96.00% <100.00%> (+0.67%)	⬆️
deepmd_cli/main.py	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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