Add a pyproject.toml

Hi,

I found it non-trivial to set up a vanilla python environment to work with `big_vision`. Would including a pyproject.toml in the root be valuable for the project? If so I could open a PR.

Here's the config that works for me:

```toml
[project]
name = "big-vision"
version = "0.1.0"
requires-python = ">=3.10,<3.11"
dependencies = [
"absl-py>=2.1.0",
"clu",
"distrax>=0.1.5",
"editdistance>=0.8.1",
"einops>=0.8.0",
"flax>=0.10.2",
"flaxformer",
"numpy<2.0.0",
"optax>=0.2.4",
"overrides>=7.7.0",
"protobuf<3.21.0",
"psutil>=6.1.1",
"pycocoevalcap>=1.2",
"sentencepiece>=0.2.0",
"tensorflow-cpu>=2.10.0",
"tensorflow-gan>=2.1.0",
"tensorflow-text>=2.9.0",
"tfds-nightly>=4.9.0.dev202304110044",
"keras<3.0.0",
"jax[cuda]>=0.4.34",
]

[tool.uv.sources]
flaxformer = { git = "https://github.com/google/flaxformer" }
clu = { git = "https://github.com/google/CommonLoopUtils" }
```

Start by reviewing the proposed root pyproject.toml configuration and its dependency constraints from the issue. Confirm that it provides a usable vanilla Python environment for big_vision; done should include a project metadata file with the required dependencies and git sources.

Written by the indexing model from the issue text.