add EmptyCacheHook and rename some modules

mmcv/runner/hooks/__init__.py

 from .hook import Hook
-from .checkpoint_saver import CheckpointHook
+from .checkpoint import CheckpointHook
 from .closure import ClosureHook
 from .lr_updater import LrUpdaterHook
-from .optimizer_stepper import OptimizerHook
+from .optimizer import OptimizerHook
 from .iter_timer import IterTimerHook
 from .sampler_seed import DistSamplerSeedHook
+from .memory import EmptyCacheHook
 from .logger import (LoggerHook, TextLoggerHook, PaviLoggerHook,
                      TensorboardLoggerHook)
 
 __all__ = [
     'Hook', 'CheckpointHook', 'ClosureHook', 'LrUpdaterHook', 'OptimizerHook',
-    'IterTimerHook', 'DistSamplerSeedHook', 'LoggerHook', 'TextLoggerHook',
-    'PaviLoggerHook', 'TensorboardLoggerHook'
+    'IterTimerHook', 'DistSamplerSeedHook', 'EmptyCacheHook', 'LoggerHook',
+    'TextLoggerHook', 'PaviLoggerHook', 'TensorboardLoggerHook'
 ]

mmcv/runner/hooks/memory.py

+import torch
+
+from .hook import Hook
+
+
+class EmptyCacheHook(Hook):
+
+    def __init__(self, before_epoch=False, after_epoch=True, after_iter=False):
+        self._before_epoch = before_epoch
+        self._after_epoch = after_epoch
+        self._after_iter = after_iter
+
+    def after_iter(self, runner):
+        if self._after_iter:
+            torch.cuda.empty_cache()
+
+    def before_epoch(self, runner):
+        if self._before_epoch:
+            torch.cuda.empty_cache()
+
+    def after_epoch(self, runner):
+        if self._after_epoch:
+            torch.cuda.empty_cache()