style: Refine name and signature of 2 replicaName functions

[houz42]

What this PR does / why we need it:

2 tiny functions in pytorch and xgboost controllers are renamed and redesigned. They both

1. do the same thing: generating names for each replica of a job
2. share same parameters: the job name, the type of the replica, an index number

So, they are renamed to replicaName, and redesigned to use proper types for parameters

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

Checklist:

• Docs included if any changes are user facing

[gaocegege]

/lgtm

/cc @kubeflow/wg-training-leads

Thanks for your contribution! 🎉 👍

[gaocegege]

/ok-to-test

[coveralls]

Pull Request Test Coverage Report for Build 3051346530

• 5 of 9 (55.56%) changed or added relevant lines in 4 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.03%) to 39.785%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pkg/controller.v1/xgboost/xgboost.go	0	4	0.0%

Files with Coverage Reduction	New Missed Lines	%
pkg/controller.v1/mpi/mpijob_controller.go	2	78.03%

Totals
Change from base Build 2973439690:	-0.03%
Covered Lines:	2329
Relevant Lines:	5854

---

💛 - Coveralls

[houz42]

/retest

[terrytangyuan]

Thanks!

/lgtm
/approve

[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: houz42, terrytangyuan

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