Barrier for Horovod KV Store API

[rbyche]

Description

Added barrier function, which was missing from Horovod KV Store API.

Checklist

Essentials

• PR's title starts with a category (e.g. [BUGFIX], [MODEL], [TUTORIAL], [FEATURE], [DOC], etc)
• Changes are complete (i.e. I finished coding on this PR)
• All changes have test coverage
• Code is well-documented

Changes

• Add _barrier function to kvstore.Horovod

Comments

• If this change is a backward incompatible change, why must this change be made.
• Interesting edge cases to note here

[mxnet-bot]

Hey @rbyche , Thanks for submitting the PR
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[rbyche]

@mxnet-bot run ci [centos-gpu, edge, miscellaneous, unix-gpu, website]

[mxnet-bot]

Jenkins CI successfully triggered : [miscellaneous, website, unix-gpu, centos-gpu, edge]