[LITE][OPENCL] support fp16 for cl_image_converter, layout, activation all OpenCL image kernel. test=develop (#2964)

* [LITE][OPENCL] support fp16 for cl_image_converter, layout, activation image kernel. test=develop

* add conv, depthwise and UT. test=develop

* add pool, conv, nearest_interp kernel. test=develop

* support fp16 for scale, reshape, concat, fc buffer opencl kernel. test=develop

* refactor for mul opencl buffer kernel. test=develop

* support fp16 for elementwise_mul opecl image kernel. test=develop

* support fp16 for elementwise_mul opencl image kernel. test=develop

* support fp16 for ele_add, fuse_ele_add_act opencl kernel. test=develop

* rename io_copy. test=develop

* mobilenetv1,v2 passed on 855. test=develop

* fix opt for opencl. test=develop

* comment All opencl buffer kernels. test=develop

* refactor conv, depthwise into one routing selection. test=develop

* [LITE][OPENCL] Add depthwise_conv_3x3 opencl kernel

* [LITE][OPENCL] Add depthwise_conv_3x3 opencl kernel. test=develop

* [LITE][OPENCL] Add Pool opencl kernel. test=develop

* fix pooling bug and speed

* fix build error

* delete VLOGin pool, test=develop

* add openmp, test=develop

* fix lite/kernels/arm/pool_compute_test basic_pooling compute error bug, test=develop

* update pooling 2-pad to 4-pad, test=develop

* fix 2-pad to 4-pad in operators/pool_op.h, AttachKernel will set param, so 2-pad to 4-pad funcs should put in AttachKernel. test=ddevellop

* put 2-pad to 4-pad in AttachImpl, test=develop

* according to reviews, fix some format error. test=develop

* fix format errorr, add (). test=develop

* change paddings type to support dynamically modify, test=develop

* update padding type int other devices, test=develop

* fix x8d build error on shared_ptr, test=ddevelop

* fix formmat in operators pool_op.cc, test=develop