more clear error msg.

theano/sandbox/cuda/basic_ops.py

@@ -2271,7 +2271,12 @@ class GpuAlloc(GpuOp):
         return hash(type(self)) ^ hash(self.memset_0)
 
     def __str__(self):
-        return "%s{memset_0=%s}" % (self.__class__.__name__, self.memset_0)
+        #Hide the memset parameter when not used to prevent confusion.
+        if self.memset_0:
+            s = "%s{memset_0=%s}" % (self.__class__.__name__, self.memset_0)
+        else:
+            s = self.__class__.__name__
+        return s
 
     def make_node(self, value, *shape):
         #if their is unneeded transfert generated by the next line