Allow specifying multiple tags with the Keras mixin

[osanseviero]

As a user, I might want to not just specify the task name, but also other tags. Since tags with a task name are automatically identified as a task, we can simply accept a list of tags .

For example

save_pretrained_keras(..., tags=["huggan", "gan", "text-to-image"])

cc @merveenoyan @NielsRogge @nateraw

[merveenoyan]

I'm currently off, will implement this tomorrow.