* [LITE][OPENCL] improve perf and support variable length for fc_buffer、elementwise_add、scale、activation、grid_sampler. test=develop

* [LITE][OPENCL] comment vlog for opencl kernel. test=develop

* fix

* fix format. test=develop

* fix format, test=develop

* fix resize error

* fix format. test=develop

* fix format, test=develop

* fix format

* note log(4) << x_image. test=develop

* note x_image printf. test=develop

* Fix act bug for opencl kernel. test=develop

* add print ActToStr. test=develop

* remove print code. test=develop

* add grid sampler

* reset act

* fix conflict and readme, test=develop

* fix ios v8 run error

* fix grid_sampler compute error. test=develop

* fix exp act run error, test=develop

* fix format, test=develop

* replace vlog with log in pool, add each run duration print in test. test=develop

* change layout file path. test=develop

* fix act, layout image kernel. test=develop

* fix spell error. test=develop

* remove clfinish in act, concat, layout, nearest_interp. test=develop

* add RELU macro define in fc opencl kernel. test=develop

* add cpu_ref print in activation opencl kernel.test=develop

* fix layout ut. test=develop

* replace log with vlog. test=develop

* fix get output. test=develop

* [LITE][OPENCL][Image] fix issue in concat and nearest_interp  thx for chenj and ys,test=develop

* [LITE][OPENCL][Image] fix issue in concat and nearest_interp  thx for chenj and ys,test=develop

[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