[Feature] Add bfloat16 support for CPU

[itaraban]

Description

Enabling BFloat16 data type for CPU.

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]

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[dgl-bot]

Commit ID: 98d990575b9ffaf571863a36b71bd3a36a4321f5

Build ID: 1

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Commit ID: 697014a9ede4e8cbbe6e83e912bef134312efc5e

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Commit ID: 97c58081f947f6930cb118056f72427874960571

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Commit ID: 81b2ca734303ab0169942e24407167c6466f3d1a

Build ID: 6

Status: ❌ CI test failed in Stage [CPU Build].

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Commit ID: 0142fe762f61c411592e604952ba266269a4201f

Build ID: 7

Status: ✅ CI test succeeded.

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[frozenbugs]

There is still one open comment, please address, overall the code style looks great! Thanks.
Add @BarclayII to double check.

[dgl-bot]

Commit ID: bb4300814fc3ebbeaa7410bacbce6aacc3a0f316

Build ID: 9

Status: ✅ CI test succeeded.

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[frozenbugs]

There is still one open comment, please address, overall the code style looks great! Thanks. Add @BarclayII to double check.

LGTM from my side, @BarclayII feel free to approve if this PR looks good to you.

[BarclayII]

Is it assuming big endian byte order? If it's little endian I think this should be

raw[0] = val;

Also, could we have a test case on converting between bfloat16 and float32?

[BarclayII]

LGTM

[dgl-bot]

Commit ID: 8b980272aa7afec62390856f0a02c97f080f25ea

Build ID: 10

Status: ✅ CI test succeeded.

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[dgl-bot]

Commit ID: 36d4f56

Build ID: 11

Status: ❌ CI test failed in Stage [Distributed Torch CPU Unit test].

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[itaraban]

@dgl-bot

[dgl-bot]

Commit ID: 2ac0771

Build ID: 12

Status: ✅ CI test succeeded.

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