add max_keep_ckpts parameter (#227)

* add max_keep_ckpts parameters to save memory when saving models

* format

* format

* format

* fixed linting error
Co-authored-by: z-bingo <z-bingo@outlook.com>

mmcv/runner/hooks/checkpoint.py

 # Copyright (c) Open-MMLab. All rights reserved.
+import os
+
 from ..dist_utils import master_only
 from .hook import HOOKS, Hook
 
@@ -10,10 +12,12 @@ class CheckpointHook(Hook):
                  interval=-1,
                  save_optimizer=True,
                  out_dir=None,
+                 max_keep_ckpts=-1,
                  **kwargs):
         self.interval = interval
         self.save_optimizer = save_optimizer
         self.out_dir = out_dir
+        self.max_keep_ckpts = max_keep_ckpts
         self.args = kwargs
 
     @master_only
@@ -25,3 +29,15 @@ class CheckpointHook(Hook):
             self.out_dir = runner.work_dir
         runner.save_checkpoint(
             self.out_dir, save_optimizer=self.save_optimizer, **self.args)
+
+        # remove other checkpoints
+        if self.max_keep_ckpts > 0:
+            filename_tmpl = self.args.get('filename_tmpl', 'epoch_{}.pth')
+            current_epoch = runner.epoch + 1
+            for epoch in range(current_epoch - self.max_keep_ckpts, 0, -1):
+                ckpt_path = os.path.join(self.out_dir,
+                                         filename_tmpl.format(epoch))
+                if os.path.exists(ckpt_path):
+                    os.remove(ckpt_path)
+                else:
+                    break