* fix PythonAPI test in Op unittest, test=develop
* fix unit tests that do not need to inherit OpTest, test=develop

* modified group_norm's unittest for pass statement, test=develop

* fix group_norm's bug: scale or bias is None which causes segmentation fault, test=develop

1. group_norm support data_layout=NHWC
2. modified doc of group_norm

* add pybind interface to get all inplace ops, test=develop

* enhance OpTest to check whether the consistency of operator when using and not using inplace, test=develop

* handle corner cases in op_test, test=develop

* support outputs without tensor holder_, like XShape in reshape_op, test=develop

* fix bug, some op has GradOpMaker, but actually no grad_op in OpInfoMap, test=develop

* use reshape_grad instead of reshape in FlattenGradOp, test=develop

* fix error debug dims info for variables like XShape, test=develop

* change computational order in sum_op to relieve computation difference using inplace, test=develop

* add inplace_atol to check group_norm, and skip inplace_grad for mkldnn, test=develop

* follow sneaxiy's comments, test=develop

* remove unused DefaultGradOpDescMaker in mkldnn op, test=develop

Add group normalization operator.