Fix test_mp_dataloader.py crashed when use graphbolt

[CfromBU]

the pr is related to the 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: 4cae7fe0992d5f6dcac0675a98eb26ee184615c3

Build ID: 1

Status: ✅ CI test succeeded.

Report path: link

Full logs path: link

[dgl-bot]

Commit ID: a2da4fc3af54512a4524cd180c240be8476364c7

Build ID: 2

Status: ⚪️ CI test cancelled due to overrun.

Report path: link

Full logs path: link

[dgl-bot]

Commit ID: d9191735a74585341b49d5e9ffc872016fa2383d

Build ID: 3

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

Report path: link

Full logs path: link