Do not import scipy.signal at torchmetrics import time

[Kayvan-Zahiri]

What does this PR do?

Part of #3457, the scipy.signal half of it. torchvision is #3314 / #3432 and matplotlib is untouched here.

import torchmetrics pulls in scipy.signal eagerly from a SRMRpy back-compatibility shim that is duplicated across torchmetrics/__init__.py, torchmetrics/audio/__init__.py and torchmetrics/functional/audio/__init__.py. The shim exists because SRMRpy calls scipy.signal.hamming, which SciPy moved to scipy.signal.windows.hamming.

torchmetrics never imports SRMRpy. It appears only in requirements/audio_test.txt and tests/unittests/audio/test_srmr.py:

$ grep -rn srmrpy src/
$ grep -rn srmrpy requirements/ tests/
requirements/audio_test.txt:7:srmrpy @ git+https://github.com/Lightning-Sandbox/SRMRpy
tests/unittests/audio/test_srmr.py:19:from srmrpy import srmr as srmrpy_srmr

So the shim belongs with the tests. It moves to tests/unittests/audio/__init__.py, which Python imports before test_srmr.py and therefore before that module's from srmrpy import .... Verified:

$ python -c "import scipy.signal; print(hasattr(scipy.signal,'hamming'))"
False
$ python -c "import sys; sys.path.insert(0,'tests'); import unittests.audio, scipy.signal; print(hasattr(scipy.signal,'hamming'))"
True

_SCIPI_AVAILABLE was imported by the two audio __init__.py files only for this block, so it goes with it.

Measurement

python -X importtime -c "import torchmetrics", three runs each, warm cache:

	run 1	run 2	run 3
before	1085 ms	1072 ms	1095 ms
after	652 ms	657 ms	655 ms

and scipy.signal is no longer in sys.modules after import torchmetrics. Roughly 40% off, which is more than scipy.signal's own subtree because its shared parents go too.

torchmetrics.audio and torchmetrics.functional.audio still import and evaluate, and ruff check / ruff format --check are clean on the four touched files.