perf: skip shuffling if there is only 1 partition

[BubbleCal]

No description provided.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 13.79310% with 25 lines in your changes missing coverage. Please review.

Project coverage is 78.71%. Comparing base (3c82243) to head (6284595).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
rust/lance-index/src/vector/v3/shuffler.rs	12.00%	22 Missing ⚠️
rust/lance-io/src/object_store.rs	0.00%	0 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3405      +/-   ##
==========================================
- Coverage   78.71%   78.71%   -0.01%     
==========================================
  Files         250      250              
  Lines       90879    90899      +20     
  Branches    90879    90899      +20     
==========================================
+ Hits        71536    71550      +14     
- Misses      16397    16404       +7     
+ Partials     2946     2945       -1     

Flag	Coverage Δ
unittests	78.71% <13.79%> (-0.01%)	⬇️

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