[Meta] Implement MetNet-3 Model

[jacobbieker]

We want to implement MetNet-3 (See #54). For this, we need to do:

• [x] #56
• [ ] #61
• [ ] Embedding for Sparse + Dense layers
• [ ] ResNet blocks
• [ ] Train script for model that matches training setup detailed in #54
• [ ] Topographical embedding layer
• [ ] Point-source to grid translator (from weather station to grid)
• [ ] Add Loss Scaling

[Raahul-Singh]

@jacobbieker I'll start with the first task!

[jacobbieker]

Great! Thanks!

[Raahul-Singh]

@Raahul-Singh to check if the layer normalisation in the MaxVit and other blocks are implemented correctly.

[shermansiu]

Note: lucidrains has open-sourced the code here: https://github.com/lucidrains/metnet3-pytorch