Remove xformers before installation

[ebr]

Summary

Try to remove xformers before installing or upgrading Invoke. This fixes the issue for users of newer GPUs where a lingering xformers installation would break torch compatibility.

If xformers is still needed (older GPU / option 2 selected for install), it will simply be reinstalled.

Related Issues / Discussions

Closes #7142

QA Instructions

1. Make a fresh install selecting InvokeAI v5.0.0 and Option 2 in the installer (CUDA+xFormers)
2. After the installation completes, activate the virtual environment and confirm that xformers is installed: pip freeze | grep xformers should return xformers==0.0.25.post1
3. Upgrade the same install to the latest release using Option 1 in the installer (no xFormers)
4. Check the virtual environment once again and confirm that xformers is no longer present (there should be no output from pip freeze | grep xformers
5. Run the application to confirm that it launches and works.

Merge Plan

Merge at will

Checklist

• The PR has a short but descriptive title, suitable for a changelog