Needed for Wan2.1 14B: support multiple GPUs

[jbilcke-hf]

Right now we cannot easily train a Wan2.1 14B, because it uses too much memory, even on a HF Space with the Nvidia A100.

But I think if we use multiple GPUs we might be able to go around this.

• Add code to manage the config for multiple GPUs
• Add UI form element to manually select the nb of GPUs
• Debug / make it work

[jbilcke-hf]

status update: I have started to implement it but it doesn't work yet