[networkx] refact clear api and implment number_of_selfloops api

[acezen]

What do these changes do?

• refact clear api by implement a ClearGraph in engine size.
• implement number_of_selfloops api and fix number_of_edges bug when graph contain selfloops

Related issue number

#62

[codecov-commenter]

Codecov Report

Merging #266 (21026b1) into main (67a18a7) will decrease coverage by 0.05%.
The diff coverage is 20.00%.

@@            Coverage Diff             @@
##             main     #266      +/-   ##
==========================================
- Coverage   78.41%   78.35%   -0.06%     
==========================================
  Files          57       57              
  Lines        5281     5286       +5     
==========================================
+ Hits         4141     4142       +1     
- Misses       1140     1144       +4     

Impacted Files	Coverage Δ
python/graphscope/framework/dag_utils.py	59.82% <20.00%> (-0.89%)	⬇️

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