Validate subgroup names are lowercase on podgroup creation

[itsomri]

What you would like to be added?

The scheduler assumes that subgroup names are lowercase: https://github.com/NVIDIA/KAI-Scheduler/blob/main/pkg/scheduler/api/podgroup_info/subgroup_info/factory.go#L127C2-L132C2
This should be validated in the admission of podgroups.

Alternatively, we can reconsider the lowercase subgroup name requirement

Why is this needed?

The lowercase name is a hidden assumption in the scheduler at the moment. We need to either remove this requirement, or validate & document it as an API.

[faizan-exe]

I think we should keep the lowercase requirement for getting consistent labelling pattern and easier matching like:

kai.scheduler/subgroup-name: "workers"

Validation should be added through admission webhooks, so users won't accidentally mismatch case between PodGroup spec and pod labels.

Also, generally the label values are lowercase.

If feasible, please assign thanks.
cc - @itsomri

[faizan-exe]

Had private discussion with @itsomri and putting it here for reference.

@itsomri had a concern:

if there's a proposal to add workload-api to k8s, which will support a basic form of gang scheduling,
In that case, we will probably integrate to it by creating matching podgroups to their API, or something similar
But what if we have this restrictions, and they don't? This will make it harder to translate their podgroups to KAI, and in some cases impossible (image they have one subgroup named Workers and one named workers - it will collide if we translate both to lowercase)

To which I tested the Workload API and by using PodGroup name having an uppercase character. It did not allow me and gave me error in screenshot attached.

Hence, we need to impose this lowercase requirement in admission webhook, an document it as an API.