[BE] Dedup hardcoded triton versions (#96580)
Define it once in `.ci/docker/trition_version.txt` and use everywhere.
Also, patch version defined in `triton/__init__.py` as currently it always returns `2.0.0` even if package name is `2.1.0`
Followup after https://github.com/pytorch/pytorch/pull/95896 where version needed to be updated in 4+ places
Pull Request resolved: https://github.com/pytorch/pytorch/pull/96580
Approved by: https://github.com/huydhn
