Add Transformer Inferer

[Warvito]

Add TransformerInferer class that is able to perform the sampling process, the forward pass, and the get probability methods for autoregressive transformers. Together with a VQVAE or a VQGAN, it should be able to receive an image and perform these operations.

[danieltudosiu]

@marksgraham are you working on this already?

[marksgraham]

No I'm just working on it for diffusion models atm. We need the transformer implemented before we can work on the transformer inferer!

[danieltudosiu]

@Warvito are we going forward on this with the design from the KCL codebase where the inferrer takes the VQ-VAE/VAE network in order to extract the compressed representation?