* Update checkpoint.py

if edit "{save_dir}/last_checkpoint" by vim, "\n" will auto append to the end of file. which will raise 

```
FileNotFoundError: [Errno 2] No such file or directory: 'save_dir/model_0060000.pth\n'
```

* Update checkpoint.py

if edit "{save_dir}/last_checkpoint" by vim, "\n" will auto append to the end of file. which will raise
```
FileNotFoundError: [Errno 2] No such file or directory: 'save_dir/model_0060000.pth\n'
```