how to get a manifold mesh before compute_uvatlas?

### Checklist

- [X] I have searched for [similar issues](https://github.com/isl-org/Open3D/issues).
- [X] For Python issues, I have tested with the [latest development wheel](https://www.open3d.org/docs/latest/getting_started.html#development-version-pip).
- [X] I have checked the [release documentation](https://www.open3d.org/docs/release/) and the [latest documentation](https://www.open3d.org/docs/latest/) (for `main` branch).

### My Question
RuntimeError: [Open3D Error] (void open3d::t::geometry::kernel::uvunwrapping::{anonymous}::ComputeUVAtlasPartition(open3d::t::geometry::TriangleMesh, float, bool, open3d::t::geometry::kernel::uvunwrapping::{anonymous}::UVAtlasPartitionOutput&)) /root/Open3D/cpp/open3d/t/geometry/kernel/UVUnwrapping.cpp:124: UVAtlasPartition: Non-manifold mesh