[Dataloading] Make loader iters iterator

[justusschock]

Description

Makes all loader iters return self in __iter_ to enable type-checking with collections.abc.Iterator
This allows checks in Libraries like PyTorch Lightning (Lightning-AI/pytorch-lightning#7164)

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

Changes

• Add __iter__ to all Iterators

[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

[BarclayII]

Hi @justusschock , I actually noticed the same issue and have a similar fix in #2878. Your fix seems simpler (I don't know PL very well) so let me give this a try.

[BarclayII]

Worked like a charm. Thanks for your contribution! I'll incorporate your changes in my examples.

[justusschock]

Hi @BarclayII, thanks for reviewing and merging. I am not that familiar with dgl, but I know PL very good. If you need help creating the examples feel free to ping me (on GH or the PL slack) :)