Bug fixings related to using validate dataset

[amcadmus]

Bug fixings:
1, when valid data is not given;
2, bug related to iterate over a dict.

Enhancement:
1, Not using ClassArg in trainer anymore
2, print name of the dataset in the summary of DeepmdDataSystem

[codecov-commenter]

Codecov Report

Merging #523 (90f744b) into devel (f49c578) will decrease coverage by 0.17%.
The diff coverage is 72.22%.

@@            Coverage Diff             @@
##            devel     #523      +/-   ##
==========================================
- Coverage   59.65%   59.48%   -0.18%     
==========================================
  Files          74       74              
  Lines        6177     6158      -19     
==========================================
- Hits         3685     3663      -22     
- Misses       2492     2495       +3     

Impacted Files	Coverage Δ
deepmd/entrypoints/train.py	23.30% <0.00%> (ø)
deepmd/utils/data_system.py	68.06% <50.00%> (ø)
deepmd/train/trainer.py	38.70% <92.30%> (-0.64%)	⬇️
source/op/_tabulate_grad.py	57.14% <0.00%> (-9.53%)	⬇️
source/op/_gelu.py	71.42% <0.00%> (-6.35%)	⬇️
deepmd/loggers/loggers.py	40.59% <0.00%> (-1.99%)	⬇️
deepmd/env.py	81.15% <0.00%> (-1.45%)	⬇️
deepmd/loss/ener.py	7.55% <0.00%> (-0.41%)	⬇️
... and 8 more

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