[Meta] Implement MetNet-3 Model

[jacobbieker]

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

• Add MaxViT blocks for interior of model #56
• Add integrating lead time into layers (i.e. FiLM, but need to make sure that the implementation we have from our MetNet-2 is correct) #61
• Embedding for Sparse + Dense layers
• ResNet blocks
• Train script for model that matches training setup detailed in Add MetNet-3 #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