* support unroll in darts
* fix bugs in optimizer; add docstring
* update darts algorithm [untested]
* modify autograd.grad to optim_wrapper.backward
* add amp in train.py; support constructor
* rename mmcls.data to mmcls.structures
* modify darts algo to support apex [not done]
* fix code spell in diff_mutable_module
* modify optim_context of dartsddp
* add testcase for dartsddp
* fix bugs of apex in dartsddp
* standardized the unittest of darts
* adapt new data_preprocessor
* fix ut bugs
* remove unness code
Co-authored-by: gaoyang07 <1546308416@qq.com>