metadata: if LICENSE exist then include a copy of it

[mofosyne]

Based on observation by @compilade in #8810 that there are companies and individuals who may want to write their own license e.g. https://huggingface.co/apple/OpenELM-270M-Instruct/blob/079458728ac069399c968eb2aba519b1411725ff/README.md?code=true#L4

So this PR will just copy any LICENSE file and adds a new kv store named general.license.content to the metadata store.

There are future opportunities to auto-match and extract SPDX if we include a python module to detect it... but I think that's asking a bit much of us. Let's see if people be willing to fill in the model card properly. This just accounts for when there is a legitimate reason to not link to an external license (ergo self written license).

• I have read the contributing guidelines
• Self-reported review complexity:
  • Low
  • Medium
  • High

[compilade]

Just so you know, there are models with a custom license without even a LICENSE file, for example, see Gemma-2 models.

I'm not sure the LICENSE content should always be included; some models do use a valid license_link.

[mofosyne]

Just so you know, there are models with a custom license without even a LICENSE file, for example, see Gemma-2 models.

I'm not sure the LICENSE content should always be included; some models do use a valid license_link.

Added check for a link before copying license file to kv store

[mofosyne]

Considering closing this PR as I'm not 100% sure of the value of this PR. Since most people would be using common/existing opensource license files which is usually provided externally via a link (or via a SPDX license name).