Add stgcn code for traffic prediction task

Description

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[ ] The PR title starts with [$CATEGORY] (such as [Model], [Doc], [Feature], [Bugfix])
[ ] Changes are complete (i.e. I finished coding on this PR)
[ ] All changes have test coverage
[ ] Code is well-documented
[ ] To my best knowledge, examples are either not affected by this change, or have been fixed to be compatible with this change
[ ] Related issue is referred in this PR