fix(mge/distributed): fix input comp_graph of broadcast operator

GitOrigin-RevId: 039fd06a933b4d1e9a74fbb5a3eaf467cc486178

python_module/megengine/distributed/functional.py

@@ -112,26 +112,20 @@ def broadcast(
         rank = get_rank()
 
     if rank == root:
-        return _collective_comm(
-            tensor,
-            key,
-            CollParam.Mode.BROADCAST,
-            nr_ranks,
-            rank,
-            root,
-            device=tensor.device,
-        )
+        inp = tensor
     else:
-        return _collective_comm(
-            get_default_graph(),
-            key,
-            CollParam.Mode.BROADCAST,
-            nr_ranks,
-            rank,
-            root,
-            dtype=tensor._symvar.dtype,
-            device=tensor.device,
-        )
+        inp = tensor._symvar.owner_graph
+
+    return _collective_comm(
+        inp,
+        key,
+        CollParam.Mode.BROADCAST,
+        nr_ranks,
+        rank,
+        root,
+        dtype=tensor.dtype,
+        device=tensor.device,
+    )
 
 
 def all_gather(