Use _dot22 instead of dot, so it is moved to gpu

theano/sandbox/cuda/basic_ops.py

@@ -2079,8 +2079,17 @@ def tensordot(a, b, axes=2):
                                 tensor.prod(a.shape[a.ndim - axes:])))
         b_reshaped = b.reshape((tensor.prod(b.shape[:axes]),
                                 tensor.prod(b.shape[axes:])))
-        return tensor.dot(a_reshaped, b_reshaped).reshape(outshape,
-                                                          ndim=outndim)
+        assert a_reshaped.ndim == 2
+        assert b_reshaped.ndim == 2
+        # We use _dot22 here because:
+        #   - we know that the number of dimensions will be 2
+        #   - it makes it possible for the computation to be moved to GPU
+        # When cuda.opt.local_gpu_tensordot is applied, it is too late
+        # for the usual blas optimizations to take place.
+        # This will change if we decide to get rid of tensor.tensordot,
+        # and always use this version.
+        return tensor.blas._dot22(a_reshaped, b_reshaped).reshape(
+                outshape, ndim=outndim)
     elif len(axes) == 2:
         # if 'axes' is a pair of axis lists, we first shuffle the axes of a and
         # b to reduce this to the first case (note the recursion).