* Fix an import error for `get_world_size` and `get_rank`

* Only import torch.distributed when needed

torch.distributed is only used in DistributedGroupSampler

* use `get_dist_info` to obtain world size and rank

`get_dist_info` from `mmcv.runner.utils` handles the problem of `distributed_c10d` doesn't exist.

* Reformat use yapf 0.27.0

* Add yapf style file

* rewrite multigpu testing with mmddp

* fix an indent

* update slurm testing script

* update readme and scripts