add a new image_size parameter in train_dalle and generate

[rom1504]

VAE models can be use with patches of any size.
For example a model trained on 16x16 patches can still be used on 32x32 patches
that increase the seq length from 256 to 1024 in dalle

This is still a draft, as I think we should not store the image_size in the vae, but rather in the dalle. Indeed dalle model need the sequence length to be of fixed size and fixed resolution, but the vae model works for any power of 2 resolution. I'll try later to do this change.

This work as-is if anyone want to experiment with it a bit.

[rom1504]

I'm thinking it should actually be possible to also make the patch_size configurable (as done in https://github.com/lucidrains/x-transformers/blob/a11b178573d2941c98a2c6d5b3a15fd9c97d4884/x_transformers/x_transformers.py#L644 )