fixed gradient bug with inplace operations

gradient.py

@@ -81,6 +81,22 @@ def grad_sources_inputs(sources, graph_inputs):
         
         output_arg = g_outputs
         input_arg = op.inputs
+
+        try:
+            dinputs = [x[0] for x in op.destroy_map().values()]
+        except AttributeError:
+            dinputs = []
+
+#        input_arg = [input in dinputs and input.copy() or input for input in input_arg]
+
+        new_input_arg = []
+        for input in input_arg:
+            if input in dinputs:
+                new_input_arg.append(input.copy())
+            else:
+                new_input_arg.append(input)
+        input_arg = new_input_arg
+        
         op_grad = op.grad(input_arg, output_arg)
         if not isinstance(op_grad, (list,tuple)):
             raise ValueError(_msg_retType, op.__class__)