[Bugfix] Wrap cub with CUB_NS_PREFIX and remove dependency on Thrust to linking issues with Torch 1.8

[nv-dlasalle]

Description

This wraps all cub usages with CUB_NS_PREFIX, and removes the one use of thrust. This prevents symbol collision when linking against PyTorch 1.8.

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

Changes

This adds /src/array/cuda/dgl_cub.cuh, which should be included in place of cub/cub.cuh, and it wraps it in the appropriate namespace.

This also changes the implementation of NonZero to use cub::DeviceSelect::If instead of thrust.

[BarclayII]

So should we avoid using thrust altogether in DGL later on?

Also a side question: if I write a PyTorch CUDA extension that uses thrust (or linking/dlopening two libraries both using thrust), would it also have symbol duplication?

EDIT: the answer is yes: pytorch/pytorch#52663

[nv-dlasalle]

@BarclayII I think until NVIDIA/thrust#1401 is resolved, it will be simplest to avoid using it in DGL.

[yzh119]

@BarclayII I think this PR fix the low-accuracy issue of dgl + cu112.

[jermainewang]

LGTM. Thanks for the fix.