sparse: set the default value of refine_factor to 1

[sparknack]

Because of the removal of forward index, the current refine searching is slower than before. To prevent performance degradation when using a non-zero value of drop_ratio_search, set the default value of refine_factor to 1.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.49%. Comparing base (3c46f4c) to head (74baa01).
Report is 291 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##           main    #1033       +/-   ##
=========================================
+ Coverage      0   73.49%   +73.49%     
=========================================
  Files         0       82       +82     
  Lines         0     7488     +7488     
=========================================
+ Hits          0     5503     +5503     
- Misses        0     1985     +1985     

see 82 files with indirect coverage changes