Defer selection of triton template (#120275)
Our prior approach to epilogue fusion was to select from a choice from a set of triton templates and extern calls based on benchmarking inputs, then unconditionally fuse epilogues. This can be sub-optimal in following ways:
- We select an extern kernel, however an epilogue like relu() exists such that choosing a triton template + relu would have been faster
- We select a triton template, epilogue fuse, and register spilling occurs causing it to be slower than not epilogue fusing.
In this PR we wait to select either the Triton Template or Extern Kernel based on benchmarking results from the kernel itself and its epilogue. As soon as a successful fusion occurs where a fused Triton Template + epilogue is faster than the unfused choice we finalize the MultiTemplateBuffer as a specific template. If no fusion occurs we'll finalize the MultiTemplateBuffer after fusion.
Note: if there are multiple epilogue fusions (not super likely), even though we select a template after the first fusion, we will still benchmark to see if subsequent epilogue are worth fusing. We could potentially defer choosing template in this case in a follow up at expense of compile time.
Gives 4% HF training win, 10% TIMM inference win. Increases compilation time which I will be trying to address more in follow up prs.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/120275
Approved by: https://github.com/jansel
ghstack dependencies: #121996
