[Feature] Support not to register lr scheduler hook (#774)

* support no lr scheduler hook

* fix lint

mmcv/runner/base_runner.py

@@ -344,7 +344,9 @@ class BaseRunner(metaclass=ABCMeta):
         self.logger.info('resumed epoch %d, iter %d', self.epoch, self.iter)
 
     def register_lr_hook(self, lr_config):
-        if isinstance(lr_config, dict):
+        if lr_config is None:
+            return
+        elif isinstance(lr_config, dict):
             assert 'policy' in lr_config
             policy_type = lr_config.pop('policy')
             # If the type of policy is all in lower case, e.g., 'cyclic',

setup.cfg

@@ -12,7 +12,7 @@ split_before_expression_after_opening_paren = true
 [isort]
 line_length = 79
 multi_line_output = 0
-known_standard_library = pkg_resources,setuptools
+known_standard_library = pkg_resources,setuptools,logging,os,warnings,abc
 known_first_party = mmcv
 known_third_party = addict,cv2,m2r,numpy,onnx,onnxoptimizer,onnxruntime,packaging,pytest,recommonmark,resnet_cifar,tensorrt,torch,torchvision,yaml,yapf
 no_lines_before = STDLIB,LOCALFOLDER