[WIP] feat: T5 support

[AlpinDale]

This PR adds support for the T5 family of models, a series of encoder-decoder models.

Currently a work in progress.

TODO:

• Add the modeling code
• Add T5Config
• Modify attention to account for the encoder
• Modify the cache engine
• Make the weight loading logic more robust
• Possibly support quantization

[db0]

No progress on this? I want to run a prompt enhancer on aphrodite 24/7 and it will give those with low-powered PCs some new way to help :)