No module named 'mmcv._ext'

This error message indicates that a module called '_ext' in the 'mmcv' package is missing. This could be caused by several reasons, such as:

- The 'mmcv' package is not installed properly.
- The version of the 'mmcv' package is not compatible with the current environment.
- The '_ext' module is not included in the 'mmcv' package.

To resolve this issue, you can try the following steps:

- Make sure that the 'mmcv' package is installed correctly. You can install it using pip or conda depending on your environment.
- Check the version of the 'mmcv' package and make sure it is compatible with your environment. You can try upgrading or downgrading the package to a compatible version.
- If the '_ext' module is missing, you can try reinstalling the 'mmcv' package with the '--no-cache-dir' option to force a fresh installation that includes all modules.
- If none of the above works, you may need to check the source code of the 'mmcv' package to see if the '_ext' module is included. If it is not, you may need to modify or patch the package to include it.