HPA support for PyTorch Elastic

[johnugeorge]

This PR adds support to PytorchJob Elastic which was introduced #1453

It has following fixes

1. Fixed LabelSelector field for Scale sub resource with correct labels formatted as string(Ref: Changing label selector type for HPA common#197)
2. Fixed Scale subresource spec path and statuspath
3. Upgraded Elastic imagenet example to latest pytorch base image.
4. Autoscaling version upgraded to v2

Fixes: #1645 #1626

/assign @gaocegege @zw0610

/cc @kubeflow/wg-training-leads

[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: johnugeorge

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[johnugeorge]

/hold

I will remove hold when review is completed

[coveralls]

Pull Request Test Coverage Report for Build 3740580459

• 6 of 28 (21.43%) changed or added relevant lines in 6 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.2%) to 39.614%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pkg/apis/kubeflow.org/v1/zz_generated.deepcopy.go	0	2	0.0%
pkg/controller.v1/paddlepaddle/paddlepaddle_controller.go	3	5	60.0%
pkg/controller.v1/pytorch/pytorchjob_controller.go	3	5	60.0%
pkg/controller.v1/xgboost/xgboostjob_controller.go	0	2	0.0%
pkg/apis/kubeflow.org/v1/openapi_generated.go	0	6	0.0%
pkg/controller.v1/pytorch/hpa.go	0	8	0.0%

Files with Coverage Reduction	New Missed Lines	%
pkg/controller.v1/mpi/mpijob_controller.go	2	77.67%

Totals
Change from base Build 3730090510:	0.2%
Covered Lines:	2691
Relevant Lines:	6793

---

💛 - Coveralls

[gaocegege]

LGTM

Thanks for your contribution! 🎉 👍

[johnugeorge]

/hold cancel

[gaocegege]

/lgtm