Add py.typed

[ValerianRey]

When using mypy, I had an issue with qpsolvers:

src/torchjd/aggregation/_utils/dual_cone.py:5: error: Skipping analyzing "qpsolvers": module is installed, but missing library stubs or py.typed marker  [import-untyped]
src/torchjd/aggregation/_utils/dual_cone.py:5: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports

I never knew about this, but PEP 561 asks package maintainers to add a py.typed file in their topmost package to indicate that the package's interface is type-hinted. Mypy will read this and analyze it only if a py.typed marker is present, or if the --follow-untyped-imports flag is provided.

So we actually have the same problem with torchjd itself! To verify this, I tried running mypy from arena, and I got the following error:

src/arena/interfaces.py:7: error: Skipping analyzing "torchjd.aggregation": module is installed, but missing library stubs or py.typed marker  [import-untyped]
src/arena/interfaces.py:7: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports

For now, when building the torchjd package, everything (included this py.typed file) is included, so I think we don't have to change anything else (this seems to be the case since setuptools 69.0.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[ValerianRey]

Note that from arena, installing this branch (add-py-typed) of torchjd seems to fix the issue:

./setup_env.sh add-py-typed
uv pip install mypy
uv run mypy src/arena

=> No more [import-untyped] error.

So it works!