Possiblitiy for deterministic results with KDTreeFlann

### 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](http://www.open3d.org/docs/latest/getting_started.html#development-version-pip).
- [X] I have checked the [release documentation](http://www.open3d.org/docs/release/) and the [latest documentation](http://www.open3d.org/docs/latest/) (for `master` branch).

### My Question

open3d.geometry.KDTreeFlann produces non-deterministic results for KNN search, also mentioned here:
> The only `non-deterministic` part of the algorithm is the `KNN` search.
I verified with [PCL implementation of `radius_outlier_removal`](https://github.com/PointCloudLibrary/pcl/blob/master/filters/include/pcl/filters/impl/radius_outlier_removal.hpp), it is similar to the Open3D implementation. Only, KD-Tree based KNN search output might differ. Open3D uses nanoflann on CPU for KNN Search.

_Originally posted by @reyanshsolis in https://github.com/isl-org/Open3D/issues/2344#issuecomment-1007505205_

As I understand o3d uses the nanoflann library. Are there any plans for possible deterministic results in the future or is this entirely up to nanoflann?

kind regards,
Andreas