[DataLoader] Allow batch_size=None for GraphDataLoader

[BarclayII]

Fixes #3358.

[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: 94d44203f1a43df3b9db71d58199f58f486c7ba9

Build ID: 1

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

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[dgl-bot]

Commit ID: 36fd21d700bcee4b2f7ca6a74869f4dcf06af361

Build ID: 2

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

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Full logs path: link

[dgl-bot]

Commit ID: 0bdab5e9966a222b3f327a6f7638c00b291466dd

Build ID: 3

Status: ✅ CI test succeeded

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Full logs path: link

[Rhett-Ying]

Commit ID: 741d55332ec87da7f8a39c41f668bd05939c914f

Build ID: 5

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

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Full logs path: link

[dgl-bot]

Commit ID: b6202e8f2053811d49ea4d771db982edfe360863

Build ID: 6

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

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Full logs path: link

[dgl-bot]

Commit ID: 9e3d8e4ededc005c617716267c0324d28d5f61b4

Build ID: 7

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

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[dgl-bot]

Commit ID: 3ed29481f789fb37d338f823de8bc7fd9e688484

Build ID: 8

Status: ✅ CI test succeeded

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[dgl-bot]

Commit ID: 79dd092f84d777452f72e833e42e0126dd30bcfd

Build ID: 9

Status: ✅ CI test succeeded

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[dgl-bot]

Commit ID: 391f641e563a5212f624d43a98e815cfc02743e4

Build ID: 11

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

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[dgl-bot]

Commit ID: a795f5207080927e4acec344334b8b2775e45e3d

Build ID: 12

Status: ✅ CI test succeeded

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