What is the meaning of subfolder "t"? What is that specifically for?

### 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

Take a look at [https://github.com/isl-org/Open3D/tree/master/cpp/open3d](https://github.com/isl-org/Open3D/tree/master/cpp/open3d) ?

There are clearly two subfolders named **io**:

[https://github.com/isl-org/Open3D/tree/master/cpp/open3d/io](https://github.com/isl-org/Open3D/tree/master/cpp/open3d/io) vs. [https://github.com/isl-org/Open3D/tree/master/cpp/open3d/t/io](https://github.com/isl-org/Open3D/tree/master/cpp/open3d/t/io)

And it looks the **FIRST** [https://github.com/isl-org/Open3D/tree/master/cpp/open3d/io](https://github.com/isl-org/Open3D/tree/master/cpp/open3d/io) is dealing with **Azure Kinect**, but the **SECOND** [https://github.com/isl-org/Open3D/tree/master/cpp/open3d/t/io](https://github.com/isl-org/Open3D/tree/master/cpp/open3d/t/io) is dealing with **Intel Realsense** ?????

If so, shouldn't these 2 sensor be at the same subfolder level????