[Bugfix] Turn off `use_alternate_stream` when TensorAdaptor is not enabled (#4987)

* turn off use_alternate_stream when tensoradaptor is not enabled

* add docstring

python/dgl/_ffi/base.py

@@ -148,3 +148,8 @@ def load_tensor_adapter(backend, version):
     if not tensor_adapter_loaded:
         logger = logging.getLogger("dgl-core")
         logger.debug("Memory optimization with PyTorch is not enabled.")
+
+
+def is_tensor_adaptor_enabled() -> bool:
+    """Check whether TensorAdaptor is enabled."""
+    return tensor_adapter_loaded

python/dgl/dataloading/dataloader.py

@@ -19,6 +19,7 @@ from torch.utils.data.distributed import DistributedSampler
 
 from ..base import NID, EID, dgl_warning, DGLError
 from ..batch import batch as batch_graphs
+from .._ffi.base import is_tensor_adaptor_enabled
 from ..heterograph import DGLGraph
 from ..utils import (
     recursive_apply, ExceptionWrapper, recursive_apply_pair, set_num_threads, get_num_threads,
@@ -821,8 +822,15 @@ class DataLoader(torch.utils.data.DataLoader):
             # Check use_alternate_streams
             if use_alternate_streams is None:
                 use_alternate_streams = (
-                    self.device.type == 'cuda' and self.graph.device.type == 'cpu' and
-                    not use_uva)
+                    self.device.type == "cuda"
+                    and self.graph.device.type == "cpu"
+                    and not use_uva
+                    and is_tensor_adaptor_enabled()
+                )
+            elif use_alternate_streams and not is_tensor_adaptor_enabled():
+                dgl_warning("use_alternate_streams is turned off because "
+                    "TensorAdaptor is not available.")
+                use_alternate_streams = False
 
         if (torch.is_tensor(indices) or (
                 isinstance(indices, Mapping) and