[distGB] change test_mp_dataloader

[CfromBU]

fix the bug in test_mp_dataloader.py when use graphbolt.
the pr is related to issue #7815

[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: 512c616

Build ID: 1

Status: ✅ CI test succeeded.

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[dgl-bot]

Commit ID: 85e9d370452fbd22058156ffc772633531ee8bb3

Build ID: 2

Status: ⚪️ CI test cancelled due to overrun.

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[dgl-bot]

Commit ID: 80339b11c1c9d2862ca662000a0207d924187fd4

Build ID: 3

Status: ✅ CI test succeeded.

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[Rhett-Ying]

have you run all tests under /tests/distributed/ in your local manually? as CI does not run them.

[dgl-bot]

Commit ID: 2cf487dc05b607507ea52420a1c3fa956e82ac74

Build ID: 4

Status: ✅ CI test succeeded.

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[dgl-bot]

Commit ID: 19c44087a524c42c3d70d25efb7cc32bb5f7845e

Build ID: 5

Status: ⚪️ CI test cancelled due to overrun.

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[dgl-bot]

Commit ID: 12debaa28a49835ea8b3542a8712c3e28e90c812

Build ID: 6

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

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[dgl-bot]

Commit ID: cc68e91a8ebb904efac8653b3dcf14fda8e41035

Build ID: 7

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

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[dgl-bot]

Commit ID: 10dae2e9ece4412a846ca5f8d94f95cd139b4540

Build ID: 8

Status: ✅ CI test succeeded.

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[Rhett-Ying]

pls make sure all distributed tests pass in your local run