Support Autoscaling replicatedJob

[tenzen-y]

What would you like to be added:
I would like to support scale subresource and the metrics corresponding to HPA resource like this:

apiVersion: jobset.x-k8s.io/v1alpha2
kind: JobSet
metadata:
  name: autoscaling-sample
spec:
  scalePolicy:
    replicatedJobName:  workers  # Job Name
    replicas: 2                  # scaling target
    autoScaling:                 # typed `[]autoscalingv2.MetricSpec`
      minReplicas: 1
      maxReplicas: 10
      metrics:
        [...] 
  replicatedJobs:
  - name: workers
[...]

Why is this needed:
In the machine learning field, we often support elastically scaling worker nodes such as PyTorch Elastic.
Here is Kubeflow TrainingOperator Example: https://github.com/kubeflow/training-operator/blob/e31d11faa9f6ce5111b60c01079d39295589e0ef/pkg/apis/kubeflow.org/v1/pytorch_types.go#L98-L135

[tenzen-y]

cc: @andreyvelich @ahg-g

[tenzen-y]

/kind feature

[danielvegamyhre]

Thanks @tenzen-y, support for scale subresource was part of the original JobSet design https://bit.ly/k8s-jobset but we haven't been able to prioritize it yet. We will likely need other developers to do the implementation here, since I am currently busy with other work.

[tenzen-y]

Thanks @tenzen-y, support for scale subresource was part of the original JobSet design https://bit.ly/k8s-jobset but we haven't been able to prioritize it yet. We will likely need other developers to do the implementation here, since I am currently busy with other work.

Yeah, actually during JobSet design in https://bit.ly/k8s-jobset, I mentioned the scale subresource :)
For sure, if anyone doesn't have enough bandwidth, I may be able to take this issue, but I'm not confident that I definitely have sufficient bandwidth for this issue now.

[danielvegamyhre]

@tenzen-y it would be helpful if you or others could take this on, a short KEP would be great so we can align on the API changes to JobSet

[tenzen-y]

@tenzen-y it would be helpful if you or others could take this on, a short KEP would be great so we can align on the API changes to JobSet

Yeah, definitely should create a small KEP. Once I find the time, I will try to assign this issue to me.

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[tenzen-y]

/remove-lifecycle stale