fix previous bug in save-model (#250)

* flush buffer when saving model

* fix bugs
Co-authored-by: xurui <xurui4@sensetime.com>

mmcv/runner/checkpoint.py

@@ -249,3 +249,4 @@ def save_checkpoint(model, filename, optimizer=None, meta=None):
     # immediately flush buffer
     with open(filename, 'wb') as f:
         torch.save(checkpoint, f)
+        f.flush()