[Bugfix] Allow pinning of empty tensor

[nv-dlasalle]

Description

Calling pin_memory_() on a DGLBlock can generate errors for cases in which there are empty views. This PR makes it so we only ensure non-empty Tensors are contiguous and not views, since it is no-op in these cases.

Checklist

Please feel free to remove inapplicable items for your PR.

• The PR title starts with [$CATEGORY] (such as [NN], [Model], [Doc], [Feature]])
• I've leverage the tools to beautify the python and c++ code.
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• 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

[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

[dgl-bot]

Commit ID: 78559d0

Build ID: 1

Status: ✅ CI test succeeded.

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