[GraphBolt] Add more detail to CopyTo docstring (#6904)

python/dgl/graphbolt/base.py

@@ -104,17 +104,41 @@ class CopyTo(IterDataPipe):
     """DataPipe that transfers each element yielded from the previous DataPipe
     to the given device. For MiniBatch, only the related attributes
     (automatically inferred) will be transferred by default. If you want to
-    transfer any other attributes, indicate them in the `extra_attrs`.
+    transfer any other attributes, indicate them in the ``extra_attrs``.
 
     Functional name: :obj:`copy_to`.
 
-    This is equivalent to
+    When ``data`` has ``to`` method implemented, ``CopyTo`` will be equivalent
+    to
 
     .. code:: python
 
        for data in datapipe:
            yield data.to(device)
 
+    For :class:`~dgl.graphbolt.MiniBatch`, only a part of attributes will be
+    transferred to accelerate the process by default:
+
+    - When ``seed_nodes`` is not None and ``node_pairs`` is None, node related
+    task is inferred. Only ``labels``, ``sampled_subgraphs``, ``node_features``
+    and ``edge_features`` will be transferred.
+
+    - When ``node_pairs`` is not None and ``seed_nodes`` is None, edge/link
+    related task is inferred. Only ``labels``, ``compacted_node_pairs``,
+    ``compacted_negative_srcs``, ``compacted_negative_dsts``,
+    ``sampled_subgraphs``, ``node_features`` and ``edge_features`` will be
+    transferred.
+
+    - Otherwise, all attributes will be transferred.
+
+    - If you want some other attributes to be transferred as well, please
+    specify the name in the ``extra_attrs``. For instance, the following code
+    will copy ``seed_nodes`` to the GPU as well:
+
+    .. code:: python
+
+       datapipe = datapipe.copy_to(device="cuda", extra_attrs=["seed_nodes"])
+
     Parameters
     ----------
     datapipe : DataPipe
@@ -122,8 +146,10 @@ class CopyTo(IterDataPipe):
     device : torch.device
         The PyTorch CUDA device.
     extra_attrs: List[string]
-        The extra attributes in the MiniBatch you want to be carried to the
-        specific device.
+        The extra attributes of the data in the DataPipe you want to be carried
+        to the specific device. The attributes specified in the ``extra_attrs``
+        will be transferred regardless of the task inferred. It could also be
+        applied to classes other than :class:`~dgl.graphbolt.MiniBatch`.
     """
 
     def __init__(self, datapipe, device, extra_attrs=None):