fix corner case crash in new Flatten.infer_shape.

theano/tensor/basic.py

@@ -5480,8 +5480,15 @@ class Flatten(Op):
 
     def infer_shape(self, node, in_shapes):
         in_shp, = in_shapes
-        out_shape = (in_shp[:self.outdim - 1] +
-                        (numpy.prod(in_shp[self.outdim - 1:]),))
+        part1 = in_shp[:self.outdim - 1]
+        part2 = in_shp[self.outdim - 1:]
+        # The if is needed as numpy.prod([]) return a float 1.0
+        # We do not want to force the other dtype to int32/64.
+        if len(part2) > 1:
+            part2 = prod(part2, dtype='int64')
+        else:
+            part2 = 1
+        out_shape = (part1 + (part2,))
         return [out_shape]
 
     def grad(self, inp, grads):