Add global replica count label/annotation to support multislice training workloads with different templates

[GiuseppeTT]

What would you like to be added:

Add a label and annotation jobset.sigs.k8s.io/global-job-replicas which will contain the total number of replicas in the JobSet.

Why is this needed:

Some users want to run multislice training workloads using replicated jobs with different templates. #617 and #649 added important features to make this possible, but the use case is still not supported because the workloads need to know the total number of replicas. The new label and annotation will finally address this.