* use global conv cache; test=develop

* use singleton cache; test=develop

* fix format error; test=develop

* add cudnn helper header; test=develop

* fix header error; test=develop

* fix mac unitest; test=develop

* fix mac unitest; test=develop

* fix file format; test=develop

* fix include file error, test=develop

* remove kernel_configs_ in class ExecutionContext and kernel_configs_map_ in class OperatorWithKernel, test=develop

* fix test_elementwise_mul_op_dim, test=develop
Co-authored-by: Nphlrain <phliuhongyu@126.com>

Change backward_guard to optimize_guard to maximize the allreduce overlap

* remove attribute in Allocator::Allocate, test=develop

* fix travis ci error, test=develop

test=develop