fix: use torch padding for kornia 0.8 compatibility

[Nianze Wu (wunianze666-netizen)]

Summary

• stop importing pad from Kornia's pyramid module
• use torch.nn.functional.pad() for the existing reflect-padding paths

Problem

Kornia 0.8.3 no longer exports pad from kornia.geometry.transform.pyramid, so the current import prevents the node module from loading. PyTorch's functional pad provides the same padding operation already used here and avoids relying on Kornia's former internal re-export.

Validation

• inspected the official kornia-0.8.3-py3-none-any.whl: pyramid.py has no exported pad function and its __all__ excludes pad
• rechecked current upstream master (ac4d998); it still imports the removed symbol
• py -m py_compile pyramid_blending.py
• git diff --check

[Nianze Wu (wunianze666-netizen)]

Revalidated this against current master (ac4d998) and the official Kornia 0.8.3 wheel: pyramid.py no longer exports pad, while the repository still imports it. This PR remains a focused two-call replacement using the already imported torch.nn.functional module. A maintainer review would be appreciated.