[Feature] Add RGCN for text classification

[hugochan]

Description

Add RGCN for text classification.

Checklist

Please feel free to remove inapplicable items for your PR.

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