[Performance Improvement] Make GPU sampling and to_block use pinned memory to decrease required synchronization

[nv-dlasalle]

Description

This uses the pinned memory from #5366 to improve GPU->CPU communicating tensor sizes during sampling and to_block. This improves the performance of UVA and GPU sampling.

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.
• The PR is complete and small, read the Google eng practice (CL equals to PR) to understand more about small PR. In DGL, we consider PRs with less than 200 lines of core code change are small (example, test and documentation could be exempted).
• 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: 8caeb728fbd5e81fb1a11416b36e441d8da877ab

Build ID: 1

Status: ❌ CI test failed in Stage [Tensorflow GPU Unit test].

Report path: link

Full logs path: link

[dgl-bot]

Commit ID: ce239062160d0e6d62c25c0af46b44f14d01ef95

Build ID: 2

Status: ❌ CI test failed in Stage [Tensorflow GPU Unit test].

Report path: link

Full logs path: link

[dgl-bot]

Commit ID: 11e3b06ae18453f479f5223fd8b28e3dc24c3ae5

Build ID: 3

Status: ✅ CI test succeeded.

Report path: link

Full logs path: link

[dgl-bot]

Commit ID: dcdefbc

Build ID: 4

Status: ✅ CI test succeeded.

Report path: link

Full logs path: link