Included changes in tensor/opt.py for #2454. These were previously lost due to…

theano/gradient.py

@@ -1881,6 +1881,8 @@ def _is_zero(x):
 class ConsiderConstant(ViewOp):
     def grad(self, args, g_outs):
         return [g_out.zeros_like(g_out) for g_out in g_outs]
+
+
 consider_constant_ = ConsiderConstant()
 
 
@@ -1916,6 +1918,7 @@ class ZeroGrad(ViewOp):
     def grad(self, args, g_outs):
         return [g_out.zeros_like(g_out) for g_out in g_outs]
 
+
 zero_grad_ = ZeroGrad()
 
 

theano/tensor/opt.py

@@ -5587,11 +5587,17 @@ else:
 # # Remove consider_constant #
 # ############################
 
-# Although the op just returns its input, it should be removed from
-# the graph to make sure all possible optimizations can be applied.
+# Although the ops ConsiderConstant, ZeroGrad and DisconnectedGrad 
+# just returns the input, it should be removed from the graph to 
+# make sure all possible optimizations can be applied.
 register_canonicalize(gof.OpRemove(theano.gradient.consider_constant_),
     'fast_compile', 'fast_run', name='remove_consider_constant')
 
+register_canonicalize(gof.OpRemove(theano.gradient.zero_grad_),
+    'fast_compile', 'fast_run', name='remove_zero_grad')
+
+register_canonicalize(gof.OpRemove(theano.gradient.disconnected_grad_),
+    'fast_compile', 'fast_run', name='remove_disconnected_grad')
 
 @register_canonicalize
 @gof.local_optimizer([theano.gradient.GradClip])