[Bug Fix] Add warning for invalid input graph of edge UDF (#2682)

* Add warning for invalid graph of edge UDF

* lint
Co-authored-by: Ubuntu <ubuntu@ip-172-31-56-220.ec2.internal>

python/dgl/core.py

@@ -2,7 +2,7 @@
 # pylint: disable=not-callable
 import numpy as np
 
-from .base import DGLError, is_all, NID, EID, ALL
+from .base import DGLError, is_all, NID, EID, ALL, dgl_warning
 from . import backend as F
 from . import function as fn
 from .frame import Frame
@@ -75,6 +75,9 @@ def invoke_edge_udf(graph, eid, etype, func, *, orig_eid=None):
     else:
         u, v = graph.find_edges(eid)
         edata = graph._edge_frames[etid].subframe(eid)
+    if len(u) == 0:
+        dgl_warning('The input graph for the user-defined edge function ' \
+                    'does not contain valid edges')
     srcdata = graph._node_frames[stid].subframe(u)
     dstdata = graph._node_frames[dtid].subframe(v)
     ebatch = EdgeBatch(graph, eid if orig_eid is None else orig_eid,