Add grad to squeeze.

theano/tensor/extra_ops.py

@@ -225,7 +225,8 @@ class SqueezeOp(theano.Op):
         z[0] = squeezed
 
     def grad(self, inputs, outputs_gradients):
-        return [None for i in inputs]
+        out = outputs_gradients[0]
+        return [out.reshape(inputs[0].shape)]
 
     def __str__(self):
         return self.__class__.__name__

theano/tensor/tests/test_extra_ops.py

@@ -125,3 +125,10 @@ class TestSqueezeOp(utt.InferShapeTester):
 
         a = np.random.random((4, 1, 2, 1))
         assert np.allclose(np.squeeze(a), f(a))
+
+    def test_grad(self):
+        x = T.dtensor4('x')
+        a = np.random.random((1, 1, 3, 4))
+
+        gf = theano.function([x], T.grad(T.sum(squeeze(x, out_nd=1)), x))
+        utt.verify_grad(SqueezeOp(out_nd=2), [a])