Make sure c2r is actually configured for backward operation

[cjolif]

Fixes #126649

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• ✅ login: cjolif / name: Christophe Jolif (87351e4)

[kulinseth]

Thanks for the PR @cjolif , can you add a test-case which this is fixing ?

[cjolif]

Sure @kulinseth will do that. Was just trying to figure out a bit more how test PyTorch test system is built to put things at the right place. If there are some docs around this you can point me to it might help accelerate my discovery.

[cjolif]

The test actually already exists:

pytorch/test/test_spectral_ops.py

Line 179 in fed536d

def test_fft_round_trip(self, device, dtype):

. However I think because of onlyNativeDeviceTypes decorator it does not run on MPS.

[cjolif]

ok @kulinseth I think I got it. I made sure the test now run on MPS. Let me know what you think?

[cjolif]

Meanwhile #125328 merged this fix (with additional fix). Still the roundtrip FFT test is not included in that fix. So I will rebase this PR and just add the roundtrip FFT test as it seems useful to avoid future regression and I don't see why it is not running on MPS.

[cjolif]

Actually #125328 seems to be breaking the test while my fix was not.

(.venv_main) ➜  pytorch git:(c2r_fft_mps_fix) ✗ pytest -s test/test_spectral_ops.py -k "test_fft_round_trip"
================================================================================================ test session starts =================================================================================================
platform darwin -- Python 3.10.14, pytest-8.2.1, pluggy-1.5.0
rootdir: /Users/cjolif/OSS/pytorch
configfile: pytest.ini
plugins: hypothesis-6.102.5
collected 542 items / 530 deselected / 12 selected
Running 12 items in this shard

test/test_spectral_ops.py .s.s..ss.sFs

====================================================================================================== FAILURES ======================================================================================================
_____________________________________________________________________________________ TestFFTMPS.test_fft_round_trip_mps_float32 _____________________________________________________________________________________
Traceback (most recent call last):
  File "/Users/cjolif/.pyenv/versions/3.10.14/lib/python3.10/unittest/case.py", line 59, in testPartExecutor
    yield
  File "/Users/cjolif/.pyenv/versions/3.10.14/lib/python3.10/unittest/case.py", line 591, in run
    self._callTestMethod(testMethod)
  File "/Users/cjolif/.pyenv/versions/3.10.14/lib/python3.10/unittest/case.py", line 549, in _callTestMethod
    method()
  File "/Users/cjolif/Personnal/Playground/.venv_main/lib/python3.10/site-packages/torch/testing/_internal/common_utils.py", line 2756, in wrapper
    method(*args, **kwargs)
  File "/Users/cjolif/Personnal/Playground/.venv_main/lib/python3.10/site-packages/torch/testing/_internal/common_device_type.py", line 419, in instantiated_test
    result = test(self, **param_kwargs)
  File "/Users/cjolif/Personnal/Playground/.venv_main/lib/python3.10/site-packages/torch/testing/_internal/common_device_type.py", line 1028, in dep_fn
    return fn(slf, *args, **kwargs)
  File "/Users/cjolif/OSS/pytorch/test/test_spectral_ops.py", line 232, in test_fft_round_trip
    self.assertEqual(x, y, exact_dtype=(
  File "/Users/cjolif/Personnal/Playground/.venv_main/lib/python3.10/site-packages/torch/testing/_internal/common_utils.py", line 3639, in assertEqual
    raise error_metas.pop()[0].to_error(
AssertionError: Tensor-likes are not close!

Mismatched elements: 66 / 67 (98.5%)
Greatest absolute difference: 3.966374397277832 at index (20,) (up to 1e-05 allowed)
Greatest relative difference: 64.65916442871094 at index (4,) (up to 1.3e-06 allowed)

To execute this test, run the following from the base repo dir:
     python test/test_spectral_ops.py -k TestFFTMPS.test_fft_round_trip_mps_float32

I will look into it more and come back here.

[cjolif]

Hmm actually I probably mixed something up. The test pass. So @kulinseth I think you can merge that one to add the roundtrip test on MPS that is skipped today.