* Fix the null ptr bug in build_cinn_pass.

* Add test for empty&ctrl var.

* Init the functions of CinnCompiler.

* Add the unit test for CinnCompiler.

* Fix some compilation errors.

* Update the UT of cinn_compiler.

* Use Decomposer&OpFusion passes in CinnCompiler::CompileGraph.

* Update some comments.

* Uncomment some includes in build_cinn_pass.cc.

* Use refs instead of ptrs as returned types of FindGraph & Compile in
CinnCompiler.

* Use the merged CinnGraphSymbolization functions in CinnCompiler.

* add feed op and new var for the generated subgraph

* perfect the test script of build_cinn_pass 

* remove useless clear and perfect some annotation

* Add CinnSubgraphSearchPass

* solve CI problem of subgraph order not same

* fix some bug by review advices

* ensure the independently of subgraph, that mean the subgraph should not have link to out-graph

* rename cinn_subgraph_search_pass to build_cinn_pass and delete paddle_to_cinn_pass

* add flag to control wheter append build cinn pass

* remove AppendPass at ParallelExecutorPassBuilder

* rename paddle_to_cinn_pass to build_cinn_pass in build_strategy and close test_run_from_cinn