* Add fusion_group_pass and elementwise pattern.

* Rewrite the detector of elementwise group.
test=develop

* Add a comment in codegen.

* Add more unittest cases.
test=develop

* Move code_generator related code to fusion_group directory.

* Correct the including path.

* Add the definition of SubGraph and finish the insert of fusion_group op in pass.

* Insert graph_vis_pass in tester to visualize the graph for debug.