[Enhancement] Replace self.file_client.get_local_path(self.ann_file) with get_local_path(self.ann_file)

[HAOCHENYE]

What's the feature?

MMEngine has refactor the fileio since version 0.2.0, and the backend can be inferred according to the format of path automatically in series of interfaces, such as get_local_path. Thanks to this improvement, we do not need to set the default argument like file_client_args=dict(type='disk')

https://github.com/search?q=repo%3Aopen-mmlab%2Fmmdetection3d+file_client_args%3Ddict&type=code

and then write code like that:

https://github.com/search?q=repo%3Aopen-mmlab%2Fmmdetection3d%20get_local_path&type=code

If we directly use the new get_local_path, users can simply set data_root as an s3 path like s3://xxx/xxx without configuring file_client_args.

Any other context?

No response

[Xiangxu-0103]

Has been refactored in #2319.