Fix pep8 errors in batch_gradient_descent.py

According to the [PEP8 guidelines](http://legacy.python.org/dev/peps/pep-0008/#indentation) this should be the correct way of indenting if one of the arguments is too long to be aligned with the opening parenthesis:
```
self._compute_grad = function(
    inputs,
    updates=updates,
    mode=self.theano_function_mode,
    name='BatchGradientDescent._compute_grad')
```