Python SDK improvements

[zhenghuiwang]

This PR improves current Python SDK:

• Adds retries to all outbound gRPC calls.
• Handles the case of logging the same artifact multiple times:
  • Case 1: the artifact already has a ID assigned. It means this artifact was saved before, then no need to save the data again.
  • Case 2: new artifact doesn't have ID, but can be considered as duplication of some artifact in the DB. For example, a model with save name, version and uri. An artifact class is asked to have a optional is_duplicated method to determine the case.
• Introduces python ABC for abstract Artifact, which formally states the protocol used before.
• add __repr__ to better print out artifact objects.

Testing:

• More tests are added.
• Rewrite the test setup to start tests from a virtual env.

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This change is 

[zhenghuiwang]

/test kubeflow-metadata-presubmit

[zhenghuiwang]

/assign @jinchihe @gaoning777

It is ready for your review. Thanks

[jinchihe]

Would please update new version in setup.py file so that we can release new pkg in Pypi for the two new big enhancements? Others look good to me.

[zhenghuiwang]

Would please update new version in setup.py file so that we can release new pkg in Pypi for the two new big enhancements? Others look good to me.

Thanks @jinchihe . setup.py is already 0.3.0. (PyPI has 0.2.0) So no need to change it.

I can publish 0.3.0 once this PR is landed.

[jinchihe]

@zhenghuiwang Great.
/lgtm

[zhenghuiwang]

/approve

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