Merge pull request #3285 from JesseLivezey/cpu_corr3

doc/library/tensor/nnet/conv.txt

@@ -120,6 +120,13 @@ TODO: Give examples on how to use these things! They are pretty complicated.
       available. To explicitly disable the graph optimizer, set
       ``THEANO_FLAGS=optimizer_excluding=conv_gemm`` in your environment.
       If using it, please see the warning about a bug in CUDA 5.0 to 6.0 below.
+    - :func:`CorrMM <theano.tensor.nnet.corr.CorrMM>`
+      This is a CPU-only 2d correlation implementation taken from
+      `caffe <https://github.com/BVLC/caffe/blob/master/src/caffe/layers/conv_layer.cpp>`_
+      and also used by Torch. It does not flip the kernel. As it provides a gradient,
+      you can use it as a replacement for nnet.conv2d. There is currently no
+      optimization to move this to GPU. This will be added when the new convolution
+      interface is finished.
     - :func:`dnn_conv <theano.sandbox.cuda.dnn.dnn_conv>` GPU-only
       convolution using NVIDIA's cuDNN library. This requires that you have
       cuDNN installed and available, which in turn requires CUDA 6.5 and a GPU