[Bug Fix] Fix undefined var bug in LegacyTUDataset

[lygztq]

Description

• Fix undefined var bug in LegacyTUDataset. The hidden_size is undefined when we use constant node features.
• Fix cache file name bug in TUDataset.

Checklist

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 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
• If the PR is for a new model/paper, I've updated the example index here.

@hetong007

[hetong007]

LGTM