Reconstruction_system unit

### 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

Hi,

I reconstruct my scene using Realsense D455 with Reconstruction_System, the unit turns out meter. I tried modifying the scale to 1 but no success. Is it possible to produce milimeter, together with the trajectory.log in milimeter scale?

Thanks