Bring o3d.t.geometry.TriangleMesh to parity with legacy TriangleMesh

[joseph-sch]

Checklist

• I have searched for similar issues.
• For Python issues, I have tested with the latest development wheel.
• I have checked the release documentation and the latest documentation (for main branch).

Proposed new feature or change

The following methods that are present in the legacy API are missing in the tensor API o3d.t.geometry.TriangleMesh:

• subdivide_midpoint()
• paint_uniform_color()
• addition operator +, and addition assignment operator +=

Moreover, the following C++ functions (at least) are not bound in Python, and would be useful:

• GetVertexPositions
• GetVertexNormals
• GetVertexColors
• GetTriangleNormals
• GetTriangleColors

I believe that making sure the tensor API does not lack anything from the legacy API is key to enable widespread transition, isn't it?

References

No response

Additional information

There is confusion among many, e.g. #6060 (comment), #6130, about the tensor vs. legacy APIs.

Also, the documentation is often lacking, e.g. the tutorial covers explicitly only the PointCloud part of the tensor Geomery API, not TriangleMesh part:

[timohl]

Moreover, the following C++ functions (at least) are not bound in Python, and would be useful:

* `GetVertexPositions`

* `GetVertexNormals`

* `GetVertexColors`

* `GetTriangleNormals`

* `GetTriangleColors`

I think those are covered by GetVertexAttr and GetTriangleAttr.
So, similar to o3d.t.geometry.PointCloud.point.positions there is o3d.t.geometry.vertex.positions.
Point cloud also does not provide explicit getters.
However, the documentation is lacking descriptions for .material, .triangle, and .vertex.

Otherwise, I agree, that the tensor stuff needs improvements.