fix: Allow multiple podSpec overrides to target the same TargetJob

[kaisoz]

What this PR does / why we need it:
Currently, it's forbidden to create a TrainJob that include PodSpecOverrides with duplicated targetJobs. This breaks the Kueue integration when there are user-provided podSpecOverrides because it injects its configuration using this API

This PR fixes this by removing such limitation

Which issue(s) this PR fixes (optional, in Fixes #<issue number>, #<issue number>, ... format, will close the issue(s) when PR gets merged):
Fixes #2871

Checklist:

• Docs included if any changes are user facing

[kaisoz]

/assign @astefanutti

PTAL! Thanks! 😊

cc @andreyvelich @tenzen-y

[coveralls]

Pull Request Test Coverage Report for Build 18312702644

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.05%) to 54.442%

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Totals
Change from base Build 18271389532:	-0.05%
Covered Lines:	1201
Relevant Lines:	2206

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💛 - Coveralls

[mimowo]

nit about title, I wouldn't call it "duplicated targetJobs", it sounds like allowing a bug.

"fix: Allow multiple podSpec overrides to target the same TargetJob"

wdyt?

[astefanutti]

/lgtm

Thanks @kaisoz!

[tenzen-y]

Thank you!
/lgtm
/approve

[google-oss-prow]

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