alt Hacker News(I'm not an expert. I'd love to be corrected by someone who actually knows.)
Floating-point arithmetic is not associative. (A+B)+C does not necessarily equal A+(B+C), but you can get a performance improvement by calculating A, B, and C in parallel, then adding together whichever two finish first. So, in theory, transformers can be deterministic, but in a real system they almost always aren't.
Not an expert either, but my understanding is that large models use quantized weights and tensor inputs for inference. Multiplication and addition of fixed-point values is associative, so unless there's an intermediate "convert to/from IEEE float" step (activation functions, maybe?), you can still build determinism into a performant model.