Hello everyone,

I am trying to build a couple of models in PyMC3 which use functions that are not readily available in pymc3.math or in theano.tensor. One simple example here is arctan . Following this thread I have built a ArcTan class which implements the function (from numpy) and the gradient, which in this case is very simple.

Here is my code:

```
import theano
import theano.tensor as tt
import theano.tests.unittest_tools
class ArcTan(tt.Op):
itypes = [tt.dscalar]
otypes = [tt.dscalar]
def perform(self, node, inputs, outputs):
x, = inputs
y = np.arctan(x)
outputs[0][0] = np.array(y)
def grad(self, inputs, g):
x, = inputs
return [1 / (1 + x ** 2) ]
```

This looks simple enough, however when I check the gradient I get the following error:

```
theano.tests.unittest_tools.verify_grad(ArcTan(), [np.array(0.2)])
GradientError: GradientError: numeric gradient and analytic gradient exceed tolerance:
At position 0 of argument 0 with shape (),
val1 = 0.961538 , val2 = 1.154266
abs. error = 0.192728, abs. tolerance = 0.000100
rel. error = 0.091090, rel. tolerance = 0.000100
Exception args:
The error happened with the following inputs:, [array(0.2)],
The value of eps is:, None,
The out_type is:, None
```

The correct value (both analytical and from numerical gradient in numpy) is the first one, 0.96. Can anyone tell me what is wrong here?