Created by: cdyangzhenyu

When use `python setup.py install`, it will report a error:

```
File "/root/miniconda3/lib/python3.7/site-packages/x2paddle-0.7.2-py3.7.egg/x2paddle/op_mapper/tf_op_mapper.py", line 88
    if self.graph.get_node(name).layer_type != "Placeholder"
                                                            ^
SyntaxError: invalid syntax
```

This commit will resolve the problem.