Extend docstring of GpuDnnTransformerGrid to include out_dims

theano/gpuarray/dnn.py

@@ -2762,6 +2762,11 @@ class GpuDnnTransformerGrid(DnnBase):
             Affine transformation tensor containing one affine transformation
             matrix per image. ``theta`` is usually generated by the localization
             network.
+
+        out_dims : tuple
+            Dimensions of the transformed inputs, containing four elements, and is given
+            by (N, C, H, W), where N is the number of inputs, C the number of channels,
+            H and W are the height and width of each input.
         """
         context_name = infer_context_name(theta)