The line is exactly the same line that's always been drawn in fair use cases.
There's absolutely nothing different whether the creator is ML or a human.
Generally, if you train an ML network to generate an almost exact copy of a thousand lines, it's obviously not fair use. If it's five simple lines, it obviously is fair use. If it's somewhere in between, there are a lot of different factors that need to be weighed in a fair use decision, which you can easily look up.
There's absolutely nothing different whether the creator is ML or a human.
Generally, if you train an ML network to generate an almost exact copy of a thousand lines, it's obviously not fair use. If it's five simple lines, it obviously is fair use. If it's somewhere in between, there are a lot of different factors that need to be weighed in a fair use decision, which you can easily look up.