[Fix] Fix bug: Resume can not work.

[kennymckormick]

The first line of cfg.text is the config filename, which will not be parsed by this line:
https://github.com/open-mmlab/mmcv/blob/de4f14e9cd547fb8bf886a362ad4f02a0be95141/mmcv/runner/base_runner.py#L346

Use cfg.pretty_text instead.

[dingfengshi]

你好，我修改成pretty_text以后还是不能resume, 还是识别不了第一行

[kennymckormick]

你好，我修改成pretty_text以后还是不能resume, 还是识别不了第一行

My bad, I didn't replace it completely, you also need to replace text in cfg.checkpoint_config.meta by pretty_text. I have raised a patch (#824) for it.
Besides, you can resume from a ckpt only if the ckpt is trained with pretty_text rather than text.