1. Support memory optimization at runtime
2. Support memory type transformation automatically at runtime.
3. Move gpu winograd convolution to Conv2D.
4. Support Data Type transformation automatically.
5. MaceTensor API support optional data type.
6. Add input_data_formats and output_data_formats at yaml
7. Related issue(#363)

1. Move all files in kernels to ops
2. unify the op_def_registry and op_registry.
3. Support op clip: do not link the ops' code when the op is not registered.

* Fix  in basic_usage.rst

You have to be inside mace dir to run git fetch