No module named 'mmcv._ext'

This error message usually occurs when the mmcv package is not installed properly or is not compatible with the current environment.

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

1. Check if mmcv is installed by running `pip list` or `conda list`. If it is not installed, install it using `pip install mmcv-full` or `conda install mmcv-full -c conda-forge`.

2. Check if the installed mmcv version is compatible with your system and other dependencies. You can check the mmcv documentation for compatibility information.

3. If you have multiple versions of mmcv installed, try uninstalling all of them and reinstalling the correct version.

4. If the issue persists, try updating your system and dependencies, and then reinstalling mmcv.

5. If none of the above steps work, try checking the mmcv GitHub repository for similar issues or post a new issue with your error message and system information.