[Model] Add DiffMG Model，created a new dataset for DiffMG called amazon

Description

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[x] The PR title starts with [$CATEGORY] (such as [NN], [Model], [Doc], [Feature]])
[x] Changes are complete (i.e. I finished coding on this PR)
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[x] Code is well-documented
[x] To the best of my knowledge, examples are either not affected by this change, or have been fixed to be compatible with this change
[x] Related issue is referred in this PR
[x] If the PR is for a new model/paper, I've updated the example index here.