set annotation automatically when EnableGangScheduling is set to true

[ChanYiLin]

To allow the gang-scheduling with using kube-batch, we need to create podGroup and set the annotation scheduling.k8s.io/group-name: $PODGROUP_NAME to each pod.

When EnableGangScheduling is set to true tf-operator create the podGroup automatically but doesn't set the annotation.
This PR fixes the problem by setting the annotation when create the pods.

fix #1031

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This change is 

[ChanYiLin]

/assign @gaocegege @richardsliu
thanks!

[coveralls]

Coverage remained the same at 76.744% when pulling c05002d on ChanYiLin:master into fd76dee on kubeflow:master.

[coveralls]

Coverage remained the same at 76.744% when pulling c05002d on ChanYiLin:master into fd76dee on kubeflow:master.

[johnugeorge]

Currently, we don't have tests for gang scheduling feature. How are these annotations used when feature is enabled? Is it a must fix?

[gaocegege]

/cc @k82cn

[ChanYiLin]

I think it's necessary.
According to gang-scheduling,

kube-batch will watch PodGroup, and the annotation scheduling.k8s.io/group-name identify which group the pod belongs to

Should I open a issue for adding the tests for gang-scheduling?

[johnugeorge]

I think it's necessary.
According to gang-scheduling,

kube-batch will watch PodGroup, and the annotation scheduling.k8s.io/group-name identify which group the pod belongs to

Should I open a issue for adding the tests for gang-scheduling?

Yes. This means that this feature is broken now ? Can you add tests ?

[wackxu]

Great, we need this, for now when we enable gang scheduling, we need set the annotation manually and it is not friendly to users.

[richardsliu]

/lgtm
Thanks for working on this. Can you open an issue for adding tests? It can be done in a separate PR.

[johnugeorge]

@wackxu Thanks for the update.

/approve

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