TEZ-4508: Allow the FAIR_PARALLELISM mode to accept multiple source vertices

[okumin]

As explained in TEZ-4508, I expect there are valid use cases where FairShuffleVertexManager with FAIR_PARALLELISM should accept multiple SCATTER_GATHER edges.

[okumin]

I expect this assertion is not must because Tez users currently use FairShuffleVertexManager in specific cases on purpose.
I guess we need to add a new DataMovementType or new FairShuffleVertexManager for JOIN, but we don't immediately need it.

[tez-yetus]

🎊 +1 overall

Vote	Subsystem	Runtime	Comment
+0 🆗	reexec	25m 59s	Docker mode activated.
		_ Prechecks _
+1 💚	dupname	0m 0s	No case conflicting files found.
+1 💚	@author	0m 0s	The patch does not contain any @author tags.
+1 💚	test4tests	0m 0s	The patch appears to include 1 new or modified test files.
		_ master Compile Tests _
+1 💚	mvninstall	17m 2s	master passed
+1 💚	compile	0m 39s	master passed with JDK Ubuntu-11.0.20+8-post-Ubuntu-1ubuntu122.04
+1 💚	compile	0m 32s	master passed with JDK Private Build-1.8.0_382-8u382-ga-1~22.04.1-b05
+1 💚	checkstyle	1m 21s	master passed
+1 💚	javadoc	0m 43s	master passed with JDK Ubuntu-11.0.20+8-post-Ubuntu-1ubuntu122.04
+1 💚	javadoc	0m 29s	master passed with JDK Private Build-1.8.0_382-8u382-ga-1~22.04.1-b05
+0 🆗	spotbugs	1m 32s	Used deprecated FindBugs config; considering switching to SpotBugs.
+1 💚	findbugs	1m 31s	master passed
		_ Patch Compile Tests _
+1 💚	mvninstall	0m 22s	the patch passed
+1 💚	compile	0m 24s	the patch passed with JDK Ubuntu-11.0.20+8-post-Ubuntu-1ubuntu122.04
+1 💚	javac	0m 24s	the patch passed
+1 💚	compile	0m 21s	the patch passed with JDK Private Build-1.8.0_382-8u382-ga-1~22.04.1-b05
+1 💚	javac	0m 21s	the patch passed
-0 ⚠️	checkstyle	0m 15s	tez-runtime-library: The patch generated 1 new + 12 unchanged - 0 fixed = 13 total (was 12)
+1 💚	whitespace	0m 0s	The patch has no whitespace issues.
+1 💚	javadoc	0m 19s	the patch passed with JDK Ubuntu-11.0.20+8-post-Ubuntu-1ubuntu122.04
+1 💚	javadoc	0m 17s	the patch passed with JDK Private Build-1.8.0_382-8u382-ga-1~22.04.1-b05
+1 💚	findbugs	0m 58s	the patch passed
		_ Other Tests _
+1 💚	unit	5m 31s	tez-runtime-library in the patch passed.
+1 💚	asflicense	0m 15s	The patch does not generate ASF License warnings.
		58m 3s

Subsystem	Report/Notes
Docker	ClientAPI=1.43 ServerAPI=1.43 base: https://ci-hadoop.apache.org/job/tez-multibranch/job/PR-306/1/artifact/out/Dockerfile
GITHUB PR	#306
JIRA Issue	TEZ-4508
Optional Tests	dupname asflicense javac javadoc unit spotbugs findbugs checkstyle compile
uname	Linux ee80f5606113 4.15.0-212-generic #223-Ubuntu SMP Tue May 23 13:09:22 UTC 2023 x86_64 x86_64 x86_64 GNU/Linux
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Default Java	Private Build-1.8.0_382-8u382-ga-1~22.04.1-b05
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checkstyle	https://ci-hadoop.apache.org/job/tez-multibranch/job/PR-306/1/artifact/out/diff-checkstyle-tez-runtime-library.txt
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Max. process+thread count	1090 (vs. ulimit of 5500)
modules	C: tez-runtime-library U: tez-runtime-library
Console output	https://ci-hadoop.apache.org/job/tez-multibranch/job/PR-306/1/console
versions	git=2.34.1 maven=3.6.3 findbugs=3.0.1
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[okumin]

While testing various cases, I found this might not work when amounts of unified vertices are uneven. I will close this one once.
https://github.com/apache/tez/blob/rel/release-0.10.3/tez-runtime-library/src/main/java/org/apache/tez/dag/library/vertexmanager/FairShuffleVertexManager.java#L443-L447