Update behavior of FromLegacy to make more flexible with materials

[errissa]

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

Current behavior of FromLegacy on a legacy TriangleMesh with more than one material is to warn the user that Tensor-based meshes don't support multiple materials and not convert any materials. Annoyingly, sometimes our importers (mostly ASSIMP) import or create dummy materials so converting those to Tensor-based TriangleMesh result in meshes with no materials. With this PR those cases will convert to a Tensor-based TriangleMesh with the first material converted as well. In most cases this should produce a mesh with a usable material. In any case, a message is output to the user warning that only the first material was converted.

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

[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.

[ssheorey]

Thanks @errissa ! Looks good with minor nits.