[TOPI] conv2d nchw gpu scheduler (#315)

* __init__ updated

* pull request updated

* build_module added

* typo fixed

* another typo fixed

* conv2d gpu scheduler for two layouts moved to tvm

* changes made according to CR

* conv2d_nchw formating updated, conv2d_hwcn tests updated

* lint error fixed

* element wise operator schedule fusing fixed for conv2d

* conv2d_nchw topi test added, all resnet workloads now pass

* conv compute lint error fixed

* fixed python 3 compatibility problem

* conv2d tensor input support added, test typo fixed, ir_pass.Simplify changed to util.get_const_int