MAGNN random metapath updated

Description

Please feel free to remove inapplicable items for your PR.

[ ] The PR title starts with [$CATEGORY] (such as [NN], [Model], [Doc], [Feature]])
[ ] Changes are complete (i.e. I finished coding on this PR)
[ ] All changes have test coverage
[ ] Code is well-documented
[ ] To the best of my 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
[ ] If the PR is for a new model/paper, I've updated the example index here.