Validate pytorchjob workers are configured when elasticpolicy is configured

[tarat44]

What this PR does / why we need it:
This PR adds a check in the Pytorchjob validating webhook to ensure that if a user configures an elastic policy that they have defined a worker spec. Previously defining an elastic policy with no worker spec was resulting in a nil pointer exception on this line. Additionally, we added validation to ensure that the worker spec defines at least one replica.

Which issue(s) this PR fixes (optional, in Fixes #<issue number>, #<issue number>, ... format, will close the issue(s) when PR gets merged):
Fixes #2278

Checklist:

• Docs included if any changes are user facing

[google-oss-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[coveralls]

Pull Request Test Coverage Report for Build 11706701329

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 11663764609:	0.0%
Covered Lines:	77
Relevant Lines:	77

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💛 - Coveralls

[tenzen-y]

@tarat44 In advance, could you sign to DCO?

https://github.com/kubeflow/training-operator/pull/2320/checks?check_run_id=32498218264

[tarat44]

@tarat44 In advance, could you sign to DCO?

https://github.com/kubeflow/training-operator/pull/2320/checks?check_run_id=32498218264

@tenzen-y Thanks for bringing this to my attention, and I apologize for the delay. I combined the commits into one and provided the sign off message.