IsMasterRole fix in pytorchjob controller

[deepanker13]

What this PR does / why we need it:

1. Correcting the function to check if the pod has a master role or not in case there is only 1 worker in the PyTorch job.

Which issue(s) this PR fixes (optional, in Fixes #<issue number>, #<issue number>, ... format, will close the issue(s) when PR gets merged):
Fixes #1961

Checklist:

• Docs included if any changes are user facing

[coveralls]

Pull Request Test Coverage Report for Build 7285263587

• 5 of 5 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.01%) to 42.896%

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Totals
Change from base Build 7243429238:	0.01%
Covered Lines:	3756
Relevant Lines:	8756

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💛 - Coveralls

[johnugeorge]

Related: #1961

[johnugeorge]

/lgtm
/approve

[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: deepanker13, johnugeorge

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[terrytangyuan]

Could we make sure that the PR titles are descriptive before merging next time?