skip multi-gpu test when running on single-gpu machine (#5186)

* skip test

* Apply suggestions from code review

Co-authored-by: Rohit Gupta <rohitgr1998@gmail.com>
Co-authored-by: Nicki Skafte <skaftenicki@gmail.com>

tests/models/test_gpu.py

@@ -47,7 +47,7 @@ def test_multi_gpu_none_backend(tmpdir):
     tpipes.run_model_test(trainer_options, model)
 
 
-@pytest.mark.skipif(not torch.cuda.is_available(), reason="test requires GPU machine")
+@pytest.mark.skipif(torch.cuda.device_count() < 2, reason="test requires multi-GPU machine")
 @pytest.mark.parametrize('gpus', [1, [0], [1]])
 def test_single_gpu_model(tmpdir, gpus):
     """Make sure single GPU works (DP mode)."""