POC for creating reproducible jax-based python environments for MaxText

Some non-obvious notes

1. The general idea of the approach is to rely on already vast and well tested locked set of deps that JAX itself uses for a big chunk of its own testing (including presubmit and continuous jobs). Propagating those dependencies inside MaxText environment ensures that for a specific JAX version MaxText runs itself on as close environment as possible to what JAX of the same version was testing itself when it was getting released.
2. To get an idea how this all works under the hood check the build_seed_env.sh, it is pretty short and self explanatory for the most part.
3. CUDA deps are pulled as python wheels, which is the recommended for JAX to get CUDA, no system-wide cuda packages are needed except driver.
4. Presense of libtpu in an env makes jax to assume that it must run on TPU, so for any GPU-based workflows libtpu must be excluded (thus the constraints_tpu_only.txt file)
5. CUDA wheels are big and heavy, installing for TPU workflows is an unnecessary waste of resources (thus the constraints_gpu_only.txt).

Quick start

0. Install uv.
1. Always start in a directory with minimal pyproject.toml (as it is in this repo), and no uv.lock file present.
2. Run ./build_seed_env.sh
3. The script above will produce maxtext_requirements_lock_3_12.txt which will contain a full set of locked maxtext python dependencies pinned to the highest version numbers available when you ran it.
4. Use maxtext_requirements_lock_3_12.txt it to set up any virtual env or Docker container you want to run MaxText in.
5. Re-running ./build_seed_env.sh at any future point in time is non-reproducible.
6. The script above also produces a pyproject.toml, which lists same dependencies as in the lock.txt but in a lower-bound form.
7. The pyproject.toml should be comitted in source tree every time it is updated (see step #1).
8. If pyproject.toml is comitted, running uv export --managed-python --locked --no-hashes --no-annotate --resolution=lowest --output-file=maxtext_requirements_lock_3_12.txt on that commit at any point in time in the future is reproducible.
9. MaxText may have different pyproject.toml (in different folders), each corresponding to a specific workflow.
10. For any commit in MaxText (assuming pyproject.toml is checked in), use command #8 to recreate MaxText Python environment for that commit.
11. To generate pyproject.toml and requirements_lock.txt for a different python version change requires-python line in pyproject.toml and pull matching jax requirements_lock_<py_ver>.txt in build_seed_env.sh repeat process from scratch (pyproject.toml should be with no deps and no uv.lock file should be present).
12. TBD: Use pyproject.toml to generate MaxText meta wheel, with all its deps lower-bounded, but not pinned.