Finufft Tests are very slow

[paquiteau]

On the local runner instance it seems that the test using finufft has a backend are running forever (more than 1hour)

to reproduce locally I ran

pytest --backend=finufft --ref=finufft --durations=10 --cov

NB: the durations arg is there to track how much time each test took

============================ slowest 10 durations ============================
69.51s call     tests/operators/test_offres_exp_approx.py::test_zmap_coeff[complex3D-numpy]
68.85s call     tests/operators/test_offres_exp_approx.py::test_b0map_coeff[real3D-numpy]
37.10s call     tests/operators/test_offres_exp_approx.py::test_zmap_coeff[complex3D-torch-cpu]
36.16s call     tests/operators/test_offres_exp_approx.py::test_b0map_coeff[real3D-torch-cpu]
17.34s call     tests/test_ndft.py::test_ndft_implicit1[grid3D]
13.86s call     tests/operators/test_offres_exp_approx.py::test_b0map_coeff[real3D-torch-gpu]
12.18s call     tests/operators/test_offres_exp_approx.py::test_b0map_coeff[real3D-cupy]
11.22s call     tests/operators/test_offres_exp_approx.py::test_zmap_coeff[complex3D-cupy]
9.25s call     tests/test_ndft.py::test_ndft_implicit2[grid3D]
7.65s call     tests/operators/test_interfaces.py::test_interface_lipschitz[random3D-finufft]

Overall, it took 10 minutes to run this command, way more reasonable than the 1h30 on the CI

[paquiteau]

From offline discussion in order to make the CI even faster:

• Extract the offres_ex_approx.py tests to not be runned with operators (They don't call any nufft actually)
• filter test_examples (e.g with commit head line)
• run examples tes in parallel (using xdist)