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Is your feature request related to a problem? Please describe.
For gr.Model3D, can show the 3d data only with face information
without face data can not show.

ex for .obj following data with face data show normal, but without "f" data can not show

Created by Open3D 
object name: output_mesh
number of vertices: 272009
number of triangles: 256444
v -0.977078 -0.860674 1.852 0.337255 0.333333 0.407843
...
f 210423//210423 211058//211058 211056//211056
...

Describe the solution you'd like
Can gr.Model3D support PointCload data directly
ex for .obj data only contain v (without face )
ex for .ply only x,y,z + r,g,b support directly like following
format binary_little_endian 1.0
comment Created by Open3D
element vertex 272009
property double x
property double y
property double z
property uchar red
property uchar green
property uchar blue
end_header

Additional context
Thank your amazing job!!