fix gh-3371, disable cudnn v3 rc version and enable it for softmax grad

theano/sandbox/cuda/dnn.py

@@ -81,20 +81,28 @@ if ((err = cudnnCreate(&_handle)) != CUDNN_STATUS_SUCCESS) {
                                          " from one version, but we link with"
                                          " a different version %s" % str(v))
                     raise RuntimeError(dnn_available.msg)
-                if version() == -1:
+                if v == -1:
                     dnn_available.avail = False
                     dnn_available.msg = (
                         "CuDNN v1 detected. This version is no longer "
                         "supported by Theano. Update your CuDNN installation "
                         "to a more recent version")
                     raise RuntimeError(dnn_available.msg)
-                if version() == (20, 20):
+                if v == (20, 20):
                     dnn_available.avail = False
                     dnn_available.msg = (
                         "You have installed a release candidate of CuDNN v2."
                         " This isn't supported anymore."
                         " Update to CuDNN v2 final version.")
                     raise RuntimeError(dnn_available.msg)
+                if v[0] >= 3000 and v[0] < 3007:
+                    # 3007 is the final release of cudnn v3
+                    dnn_available.avail = False
+                    dnn_available.msg = (
+                        "You have installed a release candidate of CuDNN v3."
+                        " This isn't supported anymore."
+                        " Update to CuDNN v3 final version.")
+                    raise RuntimeError(dnn_available.msg)
 
     return dnn_available.avail
 
@@ -2380,8 +2388,12 @@ if True:
               isinstance(node.inputs[0].owner.op, HostFromGpu)) or
              (node.inputs[1].owner and
                  isinstance(node.inputs[1].owner.op, HostFromGpu)))):
-            if not dnn_available() or version() != (2000, 2000):
+            v = version()
+            if v[0] != v[1]:
+                return
+            if not dnn_available():
                 # Softmax grad is broken in v3 rc1 for this case
+                # But we don't support cudnn v3 rc version now.
                 return
             ins = []
             for n in node.inputs: