[GraphBolt] Add read_async to gb.Feature. [1]

[mfbalin]

Description

#7540 first part.

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
• Related issue is referred in this PR
• If the PR is for a new model/paper, I've updated the example index here.

Changes

[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: b1979ad6d846a3ec0fc7e4753de1183901159c09

Build ID: 1

Status: ❌ CI test failed in Stage [Lint Check].

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[dgl-bot]

Commit ID: d84784c891e4f03227621defaf5279d6d636ce87

Build ID: 2

Status: ❌ CI test failed in Stage [Torch CPU (Win64) Unit test].

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Full logs path: link

[mfbalin]

https://dgl-ci-result.s3.us-west-2.amazonaws.com/dgl/PR-7546/2/2/logs/report.html

=========================== short test summary info ===========================
FAILED tests/python/pytorch/graphbolt/impl/test_fused_csc_sampling_graph.py::test_hetero_graph_on_shared_memory[False-False-1000-1000-10000-100000] - RuntimeError: Memory mapping failed, Win32 error: 5
=== 1 failed, 5829 passed, 861 skipped, 5173 warnings in 724.99s (0:12:04) ====

[dgl-bot]

Commit ID: 507bbd4

Build ID: 3

Status: ✅ CI test succeeded.

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[frozenbugs]

If you already returned an async_handle, why do you need to define read_async_num_stages?
Consider merging?

[mfbalin]

Because read_async returns a generator object. There is no way to also return how many stages are in it.

[frozenbugs]

You can always override the generator object to provide an extra field?

[frozenbugs]

e.g. implement the len function of the object?

[mfbalin]

How can I do that?