Improve FBCode workflow (ease and automation)

[rsyue]

Migrated to fairseq2-ext

[rsyue]

ETA: 1/15

[rsyue]

See D90216457

tldr;

• Updates previous import script logic (Sandcastle friendly, Sapling instead of hg)
• Uses CUDA 12.8 for broader compatibility (in response to #141)
• References public sentencepiece
• Autoruns CI/CD, buck build, and arc lint
• Tested and working on OnDemand

To run, use VERSION_NAME=<version> with-proxy sh ./import_fairseq2.sh

Alternatively, open an OnDemand instance in VSCode and prompt DevMate, e.g.:
Prompt: Navigate to third-party/pypi/fairseq2 and run 'VERSION_NAME=<version> import_fairseq2.sh' with 'with-proxy' enabled for network access. Monitor and report any errors. When the script is done, create a diff summarizing the changes made, tests run, and their results.

[cbalioglu]

I suggest using fairseq2-ext or another internal repository for this issue to mitigate the risk of accidentally exposing some sensitive information.