fix(sdk): Fix opsgroups dependency resolution

[mr-real]

Description of your changes:
There is a bug in the Compiler class when it resolves dependencies for Ops and OpsGroups. The nature of this bug is the following: when a dependency is an OpsGroup, compiler fails with an uncaught exception because it tries to access the wrong variable. This diff fixes this bug by changing the variable to access.

This gist surfaces the bug and proves the fix:

• With the fix execution of example.py succeeds and produces a correct yaml file.
• Without the fix the execution results in the AttributeError exception.

Checklist:

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[kubeflow-bot]

This change is 

[k8s-ci-robot]

Hi @sabiroid. Thanks for your PR.

I'm waiting for a kubeflow member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[mr-real]

@googlebot I signed it!

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We found a Contributor License Agreement for you (the sender of this pull request), but were unable to find agreements for all the commit author(s) or Co-authors. If you authored these, maybe you used a different email address in the git commits than was used to sign the CLA (login here to double check)? If these were authored by someone else, then they will need to sign a CLA as well, and confirm that they're okay with these being contributed to Google.
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[Bobgy]

/assign @Ark-kun @numerology @chensun

[Ark-kun]

/ok-to-test

[mr-real]

So, some tests seem to have failed, but I am pretty sure my change is not the reason. Are there any action items for me as for now?

[Ark-kun]

@sabiroid Thank you for this fix for the DSL compiler.
Can you comment on which scenarios are broken before the fix? I just wonder why this hasn't been triggered before.

P.S. Maybe you can add a small compiler test to the test suite.
/lgtm

[Ark-kun]

/test kubeflow-pipelines-tfx-python36

[Ark-kun]

/test kubeflow-pipelines-tfx-python36

[mr-real]

@Ark-kun Sure, it's all in the description! The broken scenario is when something depends on an OpsGroup. Here is an example Python code resulting in an exception without a fix and in correct yaml with a fix: https://gist.github.com/sabiroid/08bb7d8e5a413f7d465bbb119df5449c

Also amended the PR with the unit test :)

[mr-real]

/assign @Ark-kun

[Ark-kun]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Ark-kun

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[mr-real]

/test kubeflow-pipeline-upgrade-test
/test kubeflow-pipeline-sample-test