Import time: `scipy.signal`, `torchvision` and `matplotlib` are imported eagerly by `import torchmetrics`

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## 🚀 Feature

Don't import optional dependencies (`scipy.signal`, `torchvision`, `matplotlib`) at `import torchmetrics` time just because they happen to be installed.

### Motivation

`python -X importtime -c "import torchmetrics"` on 1.9.0 (torch 2.10, Python 3.12, warm cache), cumulative:

```
torchmetrics 3.30s
torch 1.22s
scipy.signal 0.66s
torchvision 0.85s
torchmetrics.utilities.plot 0.31s
```

Around 1.8 s of the 2.1 s that is not torch goes to three optional packages that nothing in the import path uses. Lightning pays it too, since `lightning_fabric/utilities/imports.py` calls `compare_version("torchmetrics", ...)` and that does `importlib.import_module`.

Sources, verified on `master` and 1.9.0:

`scipy.signal` comes from the SRMRpy `hamming` back-compat patch, which sits in `torchmetrics/__init__.py`, `torchmetrics/audio/__init__.py` and `torchmetrics/functional/audio/__init__.py`. All three run eagerly, so all three have to move. `srmrpy` appears only in `requirements/audio_test.txt` and `tests/unittests/audio/test_srmr.py`; the library never imports it.

`torchvision` comes from `functional/image/arniqa.py`, which imports `transforms` and `models.resnet50` at module level under `if _TORCHVISION_AVAILABLE:`. Already tracked in #3314, with open PR #3432; mentioned here only for completeness.

`matplotlib` comes from `utilities/plot.py`, which imports `matplotlib`, `matplotlib.axes` and `matplotlib.pyplot` under `if _MATPLOTLIB_AVAILABLE:`. Around 140 metric modules import it for `_AX_TYPE`, `_PLOT_OUT_TYPE`, `_CMAP_TYPE` and `style_change`.

### Pitch

Move the imports to first use, as `functional/audio/srmr.py` and `functional/image/lpips.py` already do:

- Drop the `scipy.signal` patch from the three `__init__.py` files; apply it in the audio tests or in the `Lightning-Sandbox/SRMRpy` fork if it is still needed.
- `utilities/plot.py`: make the type aliases `TYPE_CHECKING`-only and resolve `plt.style.context` inside `style_change`.
- `arniqa`: #3432.

### Alternatives

A module-level `__getattr__` (PEP 562) over the domain re-exports would cover this wholesale, at the cost of static analysis.

### Additional context

Follows #2171 (closed by #2181, #2182, #2215, #2216, #2316), which did this for the audio/detection/image/text dependencies; these paths were missed or added later.

Start with torchmetrics/__init__.py, torchmetrics/audio/__init__.py, torchmetrics/functional/audio/__init__.py, and utilities/plot.py; compare their imports with functional/audio/srmr.py and functional/image/lpips.py. Check tests/unittests/audio/test_srmr.py and requirements/audio_test.txt for the SRMRpy context. Done means optional dependencies are not imported by import torchmetrics, verified with the provided import-time command.

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