Make importable

[leoauri]

This is an attempt to make the repo importable when installed into an env.

It's not really tested, so some extra work might still be necessary. But should be 99% there, and the following definitely works:

$ uv venv
$ uv pip install -e ./s4
$ cat test/test.py
import s4.src.models.sequence.modules.pool as pool_modules
import s4.src.models.sequence.backbones.block as block_modules

print(pool_modules.__name__, block_modules.__name__)
$ uv run python test/test.py
s4.src.models.sequence.modules.pool s4.src.models.sequence.backbones.block

[leoauri]

I now made lightning an optional dep, since the explicit version tends to conflict, and added a CLI command for installing the kernels:

$ uv venv
$ uv sync
$ uv run s4-install-kernels
running build_ext
...
Kernels installed successfully!