[GraphBolt] Enable `DistributedItemSampler` test only on CPU. (#7323)

tests/python/pytorch/graphbolt/test_item_sampler.py

 import os
 import re
+import unittest
 from sys import platform
 
+import backend as F
+
 import dgl
 import pytest
 import torch
 import torch.distributed as dist
 import torch.multiprocessing as mp
 from dgl import graphbolt as gb
-from torch.testing import assert_close
 
 
 def test_ItemSampler_minibatcher():
@@ -1090,6 +1092,7 @@ def test_RangeCalculation(params):
     assert key == answer
 
 
+@unittest.skipIf(F._default_context_str != "cpu", reason="GPU not required.")
 @pytest.mark.parametrize("num_ids", [24, 30, 32, 34, 36])
 @pytest.mark.parametrize("num_workers", [0, 2])
 @pytest.mark.parametrize("drop_last", [False, True])