* 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 the definition of operation in fusion_group.

* Use operations in OperationMap to detect fusion_group of elementwise pattern.

* Add namespace fusion_group in code_generator.

* Use operations recorded in OperationMap to generate code.

* Remove implementation codes to .cc file.

* Refine Operation and CodeGenerator to make it easier to generate code for grad_op.
Refine the unittest for better reuse.

* Avoid recording the template's keyword in a array.

* Support the generating of code for grad_op and add unittest.
test=develop

* Remove replaced_element_in_order and use use number instead.
test=develop