[Sampler] Metapath sampler for metapath2vec

[BarclayII]

Description

Provides the (cyclic) metapath-based random walk sampler required by metapath2vec.

This implementation takes 857 seconds to generate a corpus of the same size as that in Metapath2vec (which takes 1563 seconds) on AMiner.

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

[jermainewang]

Overall LGTM. There are some conflicts to be resolved. Suggest using OMP to parallelize the outer loop.