Always pre-transform vertices while reading with Assimp.

[ssheorey]

Type

• Bug fix (non-breaking change which fixes an issue): Fixes #
• New feature (non-breaking change which adds functionality). Resolves #
• Breaking change (fix or feature that would cause existing functionality to not work as expected) Resolves #

Motivation and Context

While reading glb files with bone transforms (especially scaling) and a single mesh, the transform is ignored if post_process is not enabled. This can cause reading a mesh with incorrect scale factor.

e.g.: 03d693e178e54766af2905e90cbc1f03.glb file from Objaverse

Checklist:

• I have run python util/check_style.py --apply to apply Open3D code style
  to my code.
• This PR changes Open3D behavior or adds new functionality.
  • Both C++ (Doxygen) and Python (Sphinx / Google style) documentation is
    updated accordingly.
  • I have added or updated C++ and / or Python unit tests OR included test
    results (e.g. screenshots or numbers) here.
• I will follow up and update the code if CI fails.
• For fork PRs, I have selected Allow edits from maintainers.

Description

This PR adds the PreTransformVertices option to the Assimp backend call by default, so that all transforms are always applied.

Also improves error messages when reading / writing fails.

[update-docs]

Thanks for submitting this pull request! The maintainers of this repository would appreciate if you could update the CHANGELOG.md based on your changes.

[benjaminum]

lgtm

[RachithP]

Looks good!
Not sure if the failing checks are something that needs to be addressed before merging.