fixed the undefined variable problem.

theano/sandbox/multinomial.py

@@ -52,10 +52,8 @@ class MultinomialFromUniform(Op):
     def grad(self, ins, outgrads):
         pvals, unis, n = ins
         (gz,) = outgrads
-        if x.dtype in T.discrete_dtypes:
-            return [T.zeros_like(x, dtype=theano.config.floatX) for x in ins]
-        else:
-            return [T.zeros_like(x) for x in ins]
+        return [T.zeros_like(x, dtype=theano.config.floatX) if x.dtype in
+                    T.discrete_dtypes else T.zeros_like(x) for x in ins]
 
 
     def c_code_cache_version(self):