[BugFix] call g.is_homogenous instead of len(g.etypes)

[Rhett-Ying]

Description

fix for #3767. when len(g.etypes)=1 and len(g.ntypes)=2, it fails to partition graph.

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 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.

Changes

[dgl-bot]

To trigger regression tests:

• @dgl-bot run [instance-type] [which tests] [compare-with-branch];
  For example: @dgl-bot run g4dn.4xlarge all dmlc/master or @dgl-bot run c5.9xlarge kernel,api dmlc/master

[VoVAllen]

is graph len(g.etypes)=1 and len(g.ntypes)=2 a heterogenous graph or a homogenous graph?

[Rhett-Ying]

is graph len(g.etypes)=1 and len(g.ntypes)=2 a heterogenous graph or a homogenous graph?

it's hetero according to current logic of g.is_homogeneous: return len(self.ntypes) == 1 and len(self.etypes) == 1