Handle import error for rgb_to_grayscale

[BreCaspian]

Summary

Fix import error when using torchvision >= 0.15 where rgb_to_grayscale moved from
torchvision.transforms.functional_tensor to torchvision.transforms.functional.

Changes

Added backward-compatible import with fallback:

try:
    from torchvision.transforms.functional import rgb_to_grayscale  # torchvision >= 0.15
except Exception:
    from torchvision.transforms.functional_tensor import rgb_to_grayscale  # torchvision <= 0.14

Motivation

In torchvision >= 0.15, the module functional_tensor was removed.

This caused: ModuleNotFoundError: No module named 'torchvision.transforms.functional_tensor' when importing basicsr.data.degradations.

The patch keeps compatibility for both older and new versions of torchvision.

Tested Environment

Windows 11

Python: 3.9.20

Torch: 2.5.1+cu118

Torchvision: 0.20.1+cu118

Verified that import basicsr.data.degradations works on both old and new torchvision.

Validation

Ran pip install -e . and confirmed successful import of basicsr.data.degradations.

Backward compatible with torchvision <= 0.14.

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