* Add avgpool bwd reference code
* Refine naming
* Fix invalid in_element op in ref_conv
* Add example (only reference now)
* Add the full example of avgpool bwd
* Fix copyright
* Imitate MakeDescriptor from transform_conv_bwd_data_to_gemm_v1.hpp
* rename channel to c from k
* Arrange the code
* Imitate the argument from conv bwd
* Implement invoker
* Fix order of parameter in example
* Refactor reference code for different dimension
* Support different stride
* Check if argument is valid
* Fix kernel parameter for NDHWC, fastest dimension C is not reduced
* Add more data type in example
* Fix bug in example
* calculate Do Ho Wo according to the dilation
* Remove useless header
* Add comment in reference code
* Add layout parameter
* Remove layout in derived class
* Refine reference comment
