gang schedule annotation

[zlcnju]

The annotation is need to be set when use gang scheduler as described. However, now the podgroup is created by tf operator, but the annotation is not set auto. So, i think it's better to add annotation auto as the scheduler name when gang-scheduling is enabled.

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[ChanYiLin]

Thanks for the issue!

I have sent a PR to fix the issue, #1032 .
BTW, we create the podGroup with the same name as the tfJob not scheduler name. So the annotation should be scheduling.k8s.io/group-name: $PODGROUP_NAME