fix: only require barrier mode if more than one partition is trained on

[eisber]

What changes are proposed in this pull request?

only require barrier mode if more than one partition is trained on

[eisber]

/azp run

[github-actions]

Hey @eisber 👋!
Thank you so much for contributing to our repository 🙌.
Someone from SynapseML Team will be reviewing this pull request soon.

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• fix: Fix LightGBM crashes with empty partitions
• feat: Make HTTP on Spark back-offs configurable
• docs: Update Spark Serving usage
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[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[codecov-commenter]

Codecov Report

Merging #1912 (ba4856b) into master (3c09702) will decrease coverage by 0.55%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1912      +/-   ##
==========================================
- Coverage   86.81%   86.27%   -0.55%     
==========================================
  Files         301      301              
  Lines       15783    15783              
  Branches      848      842       -6     
==========================================
- Hits        13702    13616      -86     
- Misses       2081     2167      +86     

Impacted Files	Coverage Δ
.../azure/synapse/ml/vw/VowpalWabbitBaseLearner.scala	86.20% <100.00%> (ø)

... and 10 files with indirect coverage changes

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[mhamilton723]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).