Add RealPLKSR LayerNorm

[the-database]

This PR adds the LayerNorm variant of RealPLKSR, which has the following benefits over the old version which used GroupNorm:

• Better metrics. One RealPLKSR LayerNorm model was confirmed to have better metrics than the official PLKSR model.
• Better visual quality.
• Faster inference, especially when using channels last. Reduced VRAM usage as well.
• Supports FP16, just like PLKSR does (see the paper for confirmation that all training and tests for PLKSR were done in FP16). I assume supports_half was set to False due to not working with the old RealPLKSR, so I updated supports_half to be True for PLKSR and RealPLKSR LayerNorm, and False for the old RealPLKSR.