build: add make target to run distributed tests locally

[saud5150]

Follow-up to #2011, requested by @liopeer in #2011 (comment).

Description

• My change is breaking

Adds make test-distributed to run the @pytest.mark.DDP tests on the shared gloo pool locally, mirroring the CI distributed-tests job (USE_PYTEST_POOL=1 ... --runslow -m DDP) so the invocation doesn't have to be reconstructed by hand.

One non-obvious detail, documented as a comment on the target: it must use python -m pytest rather than plain pytest like the neighbouring targets. python -m puts the repo root on sys.path, which the spawned pool workers inherit; with the bare pytest entry point the workers crash on their first task with ModuleNotFoundError: No module named 'tests' and the session hangs silently at the pool's starmap. (CI never hits this because it already invokes python -m pytest.)

--runslow is kept for parity with the CI job even though neither current DDP test is slow-marked, so a future slow+DDP test behaves the same locally and in CI. On platforms without a usable backend (e.g. Windows), USE_PYTEST_POOL folds in the platform checks, so the target just reports the DDP tests as skipped.

Tests

• My change is covered by existing tests.
• My change needs new tests.
• I have added/adapted the tests accordingly.
• I have manually tested the change.

macOS arm64, Python 3.12, fresh venv (.[dev,matplotlib,minimal,timm,video]):

make test-distributed
# => 2 passed, 1 skipped, 2002 deselected in 15.70s   # BarlowTwins DDP tests run on the pool

# control — same selection with the flag off:
python -m pytest tests --runslow -m DDP
# => 3 skipped, 2002 deselected                       # DDP tests correctly skip

# regression check for the sys.path detail — bare pytest instead of python -m pytest:
USE_PYTEST_POOL=1 pytest tests --runslow -m DDP
# => workers die with ModuleNotFoundError: No module named 'tests'; session hangs

Documentation

• I have added docstrings to all public functions/methods.
• My change requires a change to the documentation ( .rst files).
• I have updated the documentation accordingly.
• The autodocs update the documentation accordingly.

Makefile-only change; the python -m pytest requirement is documented on the target itself.

Implications / comments / further issues

• .PHONY is omitted to match the neighbouring test targets (test, test-fast, lint, ...), none of which declare it. Happy to add it here (or as a repo-wide .PHONY cleanup in a separate PR) if you prefer.

[gabrielfruet]

/review

[gabrielfruet]

LGTM!! Thank you

[liopeer]

Very confused that I somehow can't run the workflows here 🤔