fix(mge/tensor): where support x or y is empty

GitOrigin-RevId: 2b69ee2e793febb7f721bf82708df73ec5dcf3d1

python_module/megengine/functional/tensor.py

@@ -353,8 +353,9 @@ def where(mask: Tensor, x: Tensor, y: Tensor) -> Tensor:
         y, mask, mode=mgb.opr_param_defs.CondTake.Mode.EQ, val=0
     )
     out = x.flatten()
-    out = mgb.opr.set_advanced_indexing(out, v0)[index0]
-    out = mgb.opr.set_advanced_indexing(out, v1)[index1]
+    index = mgb.opr.concat(index0, index1, axis=0)
+    v = mgb.opr.concat(v0, v1, axis=0)
+    out = mgb.opr.set_advanced_indexing(out, v)[index]
     out = out.reshape(x.shape)
     return out
 

python_module/test/unit/functional/test_functional.py

@@ -53,7 +53,24 @@ def test_where():
     xv1 = np.array([[1, np.inf, 2], [0, np.nan, 4], [1, 5, 7]], dtype=np.float32)
     yv1 = np.array([[5, 6, 9], [2, 7, 8], [2, 1, 9]], dtype=np.float32)
 
-    cases = [{"input": [maskv0, xv0, yv0]}, {"input": [maskv1, xv1, yv1]}]
+    cases = [
+        {"input": [maskv0, xv0, yv0]},
+        {"input": [maskv1, xv1, yv1]},
+    ]
+    opr_test(cases, F.where, ref_fn=np.where)
+
+    maskv2 = np.array([1, 1, 1], dtype=np.int32)
+    xv2 = np.array([1, 3, 2], dtype=np.float32)
+    yv2 = np.array([5, 6, 9], dtype=np.float32)
+
+    maskv3 = np.array([0, 0, 0], dtype=np.int32)
+    xv3 = np.array([1, 3, 2], dtype=np.float32)
+    yv3 = np.array([5, 6, 9], dtype=np.float32)
+
+    cases = [
+        {"input": [maskv2, xv2, yv2]},
+        {"input": [maskv3, xv3, yv3]},
+    ]
     opr_test(cases, F.where, ref_fn=np.where)