Fix install error about code style.
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.
@@ -85,7 +85,7 @@ class TFOpMapper(OpMapper):
not_placeholder = list()
for name in self.graph.input_nodes:
- if self.graph.get_node(name).layer_type != "Placeholder"
+ if self.graph.get_node(name).layer_type != "Placeholder" \
and self.graph.get_node(name).layer_type != "OneShotIterator":
not_placeholder.append(name)
for name in not_placeholder:
