Add cumprod for CNTK (keras-team#12536)

* Add `cumprod` for CNTK

* Fix PEP8

* Remove warnings

Author: taehoonlee

keras/backend/cntk_backend.py

@@ -2746,7 +2746,22 @@ def cumsum(x, axis=0):
 
 
 def cumprod(x, axis=0):
-    raise NotImplementedError
+    shape = x.shape
+    out = x
+    for rep in range(shape[axis] - 1):
+        sliced_shape = list(shape)
+        sliced_shape[axis] = rep + 1
+        if axis == 0:
+            _x = x[rep:(rep + 1)]
+        elif axis == 1:
+            _x = x[:, rep:(rep + 1)]
+        elif axis == 2:
+            _x = x[:, :, rep:(rep + 1)]
+        y = concatenate([ones(sliced_shape, dtype=x.dtype),
+                         repeat_elements(_x, rep=shape[axis] - 1 - rep, axis=axis)],
+                        axis=axis)
+        out = C.element_times(out, y)
+    return out
 
 
 def arange(start, stop=None, step=1, dtype='int32'):

tests/keras/backend/backend_test.py

@@ -554,8 +554,6 @@ def test_cumsum(self):
         check_single_tensor_operation('cumsum', (4, 2), WITH_NP)
         check_single_tensor_operation('cumsum', (4, 2), WITH_NP, axis=1)
 
-    @pytest.mark.skipif(K.backend() == 'cntk', reason='cntk does not support '
-                                                      'cumprod yet')
     def test_cumprod(self):
         check_single_tensor_operation('cumprod', (4, 2), WITH_NP)
         check_single_tensor_operation('cumprod', (4, 2), WITH_NP, axis=1)