add cosine lr schedule

mmcv/runner/hooks/lr_updater.py

 from __future__ import division
 
+from math import cos, pi
+
 from .hook import Hook
 
 
@@ -161,3 +163,20 @@ class InvLrUpdaterHook(LrUpdaterHook):
     def get_lr(self, runner, base_lr):
         progress = runner.epoch if self.by_epoch else runner.iter
         return base_lr * (1 + self.gamma * progress)**(-self.power)
+
+
+class CosineLrUpdaterHook(LrUpdaterHook):
+
+    def __init__(self, target_lr=0, **kwargs):
+        self.target_lr = target_lr
+        super(CosineLrUpdaterHook, self).__init__(**kwargs)
+
+    def get_lr(self, runner, base_lr):
+        if self.by_epoch:
+            progress = runner.epoch
+            max_progress = runner.max_epochs
+        else:
+            progress = runner.iter
+            max_progress = runner.max_iters
+        return self.target_lr + 0.5 * (base_lr - self.target_lr) * \
+            (1 + cos(pi * (progress / max_progress)))