torch compile

A chain of five elementwise operations reads and writes the tensor five times when once would do. The byte arithmetic for an unfused chain against a fused one, the launch overhead that matters at small sizes, the three fusion shapes, and the operations where fusion changes nothing because the matmul was at the roof.

Four stages: a bytecode interpreter captures a graph with guards, an autograd pass splits forward from backward, a compiler emits Triton for everything but the matmuls, and a mode that replays the step as one launch. What each buys, and the two failures that silently give it back.