Update degradations.py

[gordon0907]

Summary:
Fix #533 functional_tensor not found

The torchvision.transforms.functional_tensor module is deprecated since torchvision 0.15, and it will be no longer available in 0.17

Change import source of rgb_to_grayscale from torchvision.transforms.functional_tensor to torchvision.transforms.functional, so as to avoid the deprecation warning

[piperwolters]

Please merge this PR!!

[lightandshadow68]

This is no longer a warning. It's throwing an error. A number of libraries use BasicSR as a dependency, so patching my code isn't a solution.

[14790897]

don't worry

I released a fixed version to address this issue. Use pip install basicsr-fixed instead of pip install basicsr.