[Misc] Remove unused import. (#6358)

Co-authored-by: Ubuntu <ubuntu@ip-172-31-28-63.ap-northeast-1.compute.internal>

python/dgl/graphbolt/item_sampler.py

@@ -4,7 +4,7 @@ from collections.abc import Mapping
 from functools import partial
 from typing import Callable, Iterator, Optional
 
-from torch.utils.data import default_collate, functional_datapipe
+from torch.utils.data import default_collate
 from torchdata.datapipes.iter import IterableWrapper, IterDataPipe
 
 from ..base import dgl_warning