refactorize ExplorationTaskGroup.

[wanghan-iapcm]

• implement exploration task group in a separate file.
• provide abs method make_task. all exploration task group should implement the method.
• provide BaseExplorationTaskGroup as a data holder

[codecov]

Codecov Report

Attention: 12 lines in your changes are missing coverage. Please review.

Comparison is base (ebe7a17) 87.44% compared to head (74873cb) 87.56%.
Report is 5 commits behind head on master.

Files	Patch %	Lines
dpgen2/exploration/task/task_group.py	75.51%	12 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #187      +/-   ##
==========================================
+ Coverage   87.44%   87.56%   +0.12%     
==========================================
  Files          82       83       +1     
  Lines        4173     4183      +10     
==========================================
+ Hits         3649     3663      +14     
+ Misses        524      520       -4     

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