[Feature] Support unroll with MMDDP in darts algorithm (open-mmlab#210)

* 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>

configs/nas/mmcls/darts/darts_supernet_unroll_1xb96_cifar10.py

@@ -33,8 +33,11 @@
     _delete_=True,
     constructor='mmrazor.SeparateOptimWrapperConstructor',
     architecture=dict(
+        type='OptimWrapper',
         optimizer=dict(type='SGD', lr=0.025, momentum=0.9, weight_decay=3e-4),
         clip_grad=dict(max_norm=5, norm_type=2)),
-    mutator=dict(optimizer=dict(type='Adam', lr=3e-4, weight_decay=1e-3)))
+    mutator=dict(
+        type='OptimWrapper',
+        optimizer=dict(type='Adam', lr=3e-4, weight_decay=1e-3)))
 
 find_unused_parameter = False