fix(mge/functional): fix expand_dims for scalar

GitOrigin-RevId: 253ea608f7e45a86a90e53cf6159964b2ab54678

imperative/python/megengine/functional/tensor.py

@@ -851,7 +851,14 @@ def expand_dims(inp: Tensor, axis: Union[int, Sequence[int]]) -> Tensor:
     axis = get_axes()
     ndim = inp.ndim + len(axis)
     axis = sorted(i + ndim if i < 0 else i for i in axis)
-
+    assert axis, "axis could not be empty"
+    if inp._isscalar():
+        assert axis[0] == 0, "invalid axis {} for ndim 0".format(axis[0])
+        if len(axis) == 1:
+            inp = copy(inp, device=None)
+            inp._unsetscalar()
+            return inp
+        axis = axis[1:]
     op = builtin.AddAxis(axis=axis)
     (result,) = apply(op, inp)
     return result

imperative/python/test/unit/functional/test_tensor.py

@@ -253,6 +253,19 @@ def test_expand_dims(is_varnode):
         np.testing.assert_equal(y, yy.numpy())
 
 
+def test_expand_dims_for_scalar():
+    x = np.array(1, dtype="float32")
+    xx = make_tensor(x, None)
+    for axis in [0, -1, (0, 1), (-1, -2), (0, -1)]:
+        y = np.expand_dims(x, axis)
+        yy = F.expand_dims(xx, axis)
+        np.testing.assert_equal(y, yy.numpy())
+
+    for axis in [1, -2, (1, 2), (-2, -3)]:
+        np.testing.assert_raises(np.AxisError, np.expand_dims, x, axis)
+        np.testing.assert_raises(AssertionError, F.expand_dims, xx, axis)
+
+
 @pytest.mark.parametrize("is_varnode", [True, False])
 def test_elemwise_dtype_promotion(is_varnode):
     if is_varnode: