ci: run distributed pool tests in a dedicated gloo job

[saud5150]

Part of #1982.

Description

• My change is breaking

The distributed test pool from #1983 is gated behind USE_PYTEST_POOL, which defaults to off, so its @pytest.mark.DDP tests never run in CI — they only execute when the flag is set manually. This adds a dedicated distributed-tests job that runs the DDP tests on the shared gloo (CPU) pool with USE_PYTEST_POOL=1, so real multi-rank collectives are exercised on every PR instead of relying on mocks (the gap that hid #1977 until #1980).

The flag stays off by default everywhere else, so local runs and the main 3.7/3.12 test matrix are unchanged — only this job enables the pool. It's a separate job (not a step in the existing test job) so a pool issue can't destabilise the main suite, and it runs on Python 3.12, the version I verified on Linux (below).

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.

Before enabling it, I reproduced the CI runner in Docker (python:3.12-slim, CPU torch, gloo) to confirm the spawn path runs green on Linux — Linux defaults multiprocessing to fork, but the pool forces spawn, so it behaves like macOS:

# flag ON — exact CI command form
USE_PYTEST_POOL=1 python -m pytest -s -v --runslow -m DDP tests/loss/
  => 2 passed, 1055 deselected      # both BarlowTwins DDP tests run on the pool

# flag OFF (control)
python -m pytest tests/loss/test_barlow_twins_loss.py
  => 4 passed, 2 skipped            # DDP tests correctly skip

The job's collection scope matches the existing test job (whole repo, --ignore=./lightly/openapi_generated/), which already proves collection works with the full pinned deps.

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.

CI-only change — no public API or docs.

Implications / comments / further issues

A few decisions I'd value your call on:

• 3.12 only for now (the version I verified on Linux); happy to add 3.7 if you want it covered.
• Dedicated job vs. a step in the existing 3.12 test job — I chose a separate job for isolation; a step would be smaller but couples pool flakiness to the main suite.
• If you make this a required check, it's skipped on docs/examples/benchmarks-only PRs (same detect-code-changes gate as test), so it needs the same required-but-skippable handling — related to ci: run required checks for merge groups #2007.

Unrelated side-finding while testing on torch 2.13: torch.distributed.nn.functional.all_reduce (used by the #1980 BarlowTwins fix) now emits a FutureWarning (deprecated for torch.distributed._functional_collectives.all_reduce). Not touched here — flagging as a possible follow-up.