Fix the group name, keep it consistent with the CRD definition

[runzhen]

What this PR does / why we need it:

The CRD defines the GROUP name is "trainer.kubeflow.org", but in constants.py, GROUP is still "kubeflow.org".
It will cause an error when using the training client.

Checklist:

• Docs included if any changes are user facing

[google-oss-prow]

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[runzhen]

Hi @andreyvelich, could you review or approve this PR ?

[andreyvelich]

Thank you for this fix!
I think, we've already change it in this PR: #2415 (comment)

[runzhen]

@andreyvelich yes, I didn't see your PR. Let me close this one, thanks !