sparse: make bm25 avgdl support value smaller than 1

[sparknack]

issue: #1035

[mergify]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 72.08%. Comparing base (3c46f4c) to head (029b204).
Report is 317 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##           main    #1077       +/-   ##
=========================================
+ Coverage      0   72.08%   +72.08%     
=========================================
  Files         0       86       +86     
  Lines         0     8206     +8206     
=========================================
+ Hits          0     5915     +5915     
- Misses        0     2291     +2291     

see 86 files with indirect coverage changes

[zhengbuqian]

/lgtm
/approve

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[APPROVALNOTIFIER] This PR is APPROVED

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[zhengbuqian]

issue: milvus-io/milvus#39583

/kind fix

[sre-ci-robot]

@zhengbuqian: The label(s) kind/fix cannot be applied, because the repository doesn't have them.

In response to this:

issue: milvus-io/milvus#39583

/kind fix

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[zhengbuqian]

/kind bug

[zhengbuqian]

issue: milvus-io/milvus#39583

[sparknack]

issue: #1035