ci: Test Python 3.15 with torch nightly

[mattbuot]

• Related to Add Python 3.15 support #779

This is a first step towards Python 3.15 support: it runs the tests on Python 3.15 in the CI, using torch nightly since stable torch doesn't ship cp315 wheels yet. It doesn't make support official (no classifier, no change to installation.md, no changelog entry). I thought that would be better done in a follow-up, like #474, once stable torch supports 3.15. Happy to change the scope if you'd prefer it another way.

• Add a Python 3.15 + torch nightly config to the tests matrix, with the quadprog_projector and fairgrad options. cvxpy (needed by cagrad and nash_mtl) is left out for now: its dependency sparsediffpy has no cp315 wheel yet, and building it from source on ubuntu-latest fails because BLAS isn't installed. Once wheels are available, this could switch to full.
• In install-deps, install torch nightly before the other dependencies instead of upgrading to it afterwards. Otherwise, the first install fails on Python 3.15 because no stable torch wheel matches. The existing torch-nightly config should end up with the same packages as before.
• Rename tests/profiling to tests/profiler (and PROFILING_RESULTS_DIR to PROFILER_RESULTS_DIR). Python 3.15 adds a stdlib profiling package (PEP 799), and since tests/ is on sys.path during the tests, tests/profiling shadowed it. This broke import cProfile, and therefore torch._dynamo and torchvision, so test collection failed.

Local checks, following CONTRIBUTING.md:

• Python 3.15.0rc2 + torch 2.15.0.dev20260929, without options: 2737 passed, 79 skipped, 33 xfailed.
• Python 3.15.0rc2 + torch nightly with [quadprog_projector,fairgrad]: 3140 passed and 1 failed (with [full], which builds on macOS thanks to Accelerate: 3280 passed and the same 1 failed): test_upgrad.py::test_permutation_invariant[UPGrad-T[20, 100]1], which is off by 5.6e-7 against a 5e-7 tolerance. I get the same failure on Python 3.14 + torch nightly on my machine (macOS arm64), and the torch-nightly job on main is green on Linux, so I believe it's platform-specific and unrelated to Python 3.15.
• Python 3.14 + stable torch, [full]: pytest -W error tests/unit passes, and ty check, pre-commit run --all-files, make doctest and make html pass too.

🤖 Generated with Claude Code

https://claude.ai/code/session_01LnhiXDUDYQhk7YvfpMC61d

[ValerianRey]

• Remove tests/profiling/__init__.py. Python 3.15 adds a stdlib profiling package (PEP 799), and since tests/ is on sys.path during the tests, tests/profiling shadowed it. This broke import cProfile, and therefore torch._dynamo and torchvision, so test collection failed. Without the __init__.py, tests/profiling is only a namespace package, so the stdlib one takes precedence. The profiling scripts are run as scripts and aren't imported anywhere, so nothing else depends on it. If you'd rather rename the folder instead, I can do that.

Yeah, could we instead rename profiling to profiler? Make sure to rename all mentions of it.

Apart from that I think we should merge this already.

[mattbuot]

Rename done ✅
Thanks for the quick review, let me know if you'd want any other change

[ValerianRey]

@PierreQuinton fyi