Fix np ndarray mul varbase (#24331)

* fix numpy ndarray mul var base error; test=develop

* add comment for __array_ufunc__ ; test=develop

* move unitest from imperative math op path to test_math_op_patch_var_base;
test=develop

python/paddle/fluid/dygraph/math_op_patch.py

@@ -249,3 +249,13 @@ def monkey_patch_math_varbase():
     core.VarBase.__len__ = _len_
     core.VarBase.__index__ = _index_
     core.VarBase.astype = astype
+    """
+    When code is written like this
+    y = np.pi * var
+    ndarray.__mul__(self, var) is called, var will be traced as an array(by using __len__, __getitem__), which is not right.
+    when var.__array_ufunc__  is set to None, var.__rmul__(self,  np) will be called.
+
+    The details can be seen bellow:
+    https://docs.scipy.org/doc/numpy-1.13.0/neps/ufunc-overrides.html#behavior-in-combination-with-python-s-binary-operations
+    """
+    core.VarBase.__array_ufunc__ = None

python/paddle/fluid/tests/unittests/test_math_op_patch_var_base.py

@@ -237,6 +237,20 @@ class TestMathOpPatchesVarBase(unittest.TestCase):
             str1 = "just test"
             self.assertTrue(str1[var1] == 's')
 
+    def test_np_left_mul(self):
+        with fluid.dygraph.guard():
+            t = np.sqrt(2.0 * np.pi)
+            x = fluid.layers.ones((2, 2), dtype="float32")
+            y = t * x
+
+            self.assertTrue(
+                np.allclose(
+                    y.numpy(),
+                    t * np.ones(
+                        (2, 2), dtype="float32"),
+                    rtol=1e-05,
+                    atol=0.0))
+
 
 if __name__ == '__main__':
     unittest.main()