perf: don't concat the batches when merging partitions

[BubbleCal]

then we can save much memory and time

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 72.72727% with 3 lines in your changes missing coverage. Please review.

Project coverage is 78.64%. Comparing base (aae351b) to head (d1b394b).

Files with missing lines	Patch %	Lines
rust/lance/src/index/vector/builder.rs	72.72%	0 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3410      +/-   ##
==========================================
- Coverage   78.69%   78.64%   -0.05%     
==========================================
  Files         250      250              
  Lines       90899    90903       +4     
  Branches    90899    90903       +4     
==========================================
- Hits        71532    71495      -37     
- Misses      16424    16468      +44     
+ Partials     2943     2940       -3     

Flag	Coverage Δ
unittests	78.64% <72.72%> (-0.05%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.