* Enable generating code for a given subgraph.

* Support sorting the subgraph.

* Remove the rearange of expressions because we use the sorted subgraph directly.

* Enable generating code for a subgraph which is composed of grad ops.

* Use expression information to check the accuracy in unittest.

* Separate load and store from computation expressions.
test=develop

* Improve the loading statements in generated codes.
test=develop

* Remove unused arguments from formal list.
test=develop

* 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.