add warnings when radius or radius_graph are called on CPU with `…

…max_num_neighbors` (#9076)

Warn users of the issue described
[here](#9036).

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: rusty1s <matthias.fey@tu-dortmund.de>

torch_geometric/nn/pool/__init__.py

@@ -218,6 +218,13 @@ def radius(
             Automatically calculated if not given. (default: :obj:`None`)
 
     :rtype: :class:`torch.Tensor`
+
+    .. warning::
+
+        The CPU implementation of :meth:`radius` with :obj:`max_num_neighbors`
+        is biased towards certain quadrants.
+        Consider setting :obj:`max_num_neighbors` to :obj:`None` or moving
+        inputs to GPU before proceeding.
     """
     if not torch_geometric.typing.WITH_TORCH_CLUSTER_BATCH_SIZE:
         return torch_cluster.radius(x, y, r, batch_x, batch_y,
@@ -268,6 +275,13 @@ def radius_graph(
             Automatically calculated if not given. (default: :obj:`None`)
 
     :rtype: :class:`torch.Tensor`
+
+    .. warning::
+
+        The CPU implementation of :meth:`radius_graph` with
+        :obj:`max_num_neighbors` is biased towards certain quadrants.
+        Consider setting :obj:`max_num_neighbors` to :obj:`None` or moving
+        inputs to GPU before proceeding.
     """
     if batch is not None and x.device != batch.device:
         warnings.warn("Input tensor 'x' and 'batch' are on different devices "