handle_legacy_interface raises warning when passing weights=SomeString

[NicolasHug]

handle_legacy_interface raises a warning when passing weights as strings. This leads to warnings in the training references which all rely on strings rather than weight instances.

from torchvision.prototype.models import resnet18, ResNet18_Weights
resnet18(weights="ResNet18_Weights.ImageNet1K_V1")  # warning
resnet18(weights=ResNet18_Weights.ImageNet1K_V1)  # no warning, all good

The warning is

 other than a weight enum or `None` for 'weights' are deprecated. The current behavior is equivalent to passing `weights=ResNet18_Weights.ImageNet1K_V1`. You can also use `weights=ResNet18_Weights.default` to get the most up-to-date weights.

CC @pmeier @datumbox