[Performance] Remove phmap dependency.

[mfbalin]

Description

@frozenbugs I went ahead and dropped phmap as a dependency as you requested. Please let me know if the PR has can be improved. I am merging so that I can delete the phmap folder from my repo.

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: fc5da8a

Build ID: 1

Status: ⚪️ CI test cancelled due to overrun.

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[dgl-bot]

Commit ID: f6f37e0

Build ID: 2

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

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[dgl-bot]

Commit ID: 83f579b

Build ID: 3

Status: ✅ CI test succeeded.

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[frozenbugs]

LGTM, please keep an eye on next regression result, which will be send later today.

[Rhett-Ying]

why wrap the value access with a function? Is it necessary? It's different from std::unordered_map?

[mfbalin]

Yes, it->second is const with this map implementation by default. Use .value() to modify it. I wrapped it so that when we change map again for any reason, we have to only change this function.