Track exact origin_node on best effort basis (#100110)
Currently, we track 'origins' on IR nodes so that we have some idea about what FX IR nodes contributed to any given fused kernel. However, the origins are dumped into an undifferentiated set, so if you have, e.g., multiple outputs, you cannot easily tell which output corresponds to which FX node.
This PR introduce a more precise notion of tracking "origin_node" which says that the contents of this Buffer/Loop node corresponds EXACTLY to the output of a particular FX node; e.g., if you serialized each intermediate when running the generated inductor code, you could compare them with the corresponding intermediates from the original FX graph.
Tracking origin_node in all cases requires quite a bit of effort, so this PR introduces the tracking on a strictly best effort basis. The logic in torch/_inductor/graph.py sets up the associations, but only when it is "obvious" which IR node should get the assignment, and there is work in torch/_inductor/ir.py for propagating this information around as necessary. Like origins, origin_node is not a true dataclass field (as this would break all existing positional arg call sites), instead, it is added post facto via `__post_init__`. At the moment, it is only valid for Buffer/Loop to have an origin_node, but we could imagine relaxing this in the future.
The payoff is in torch/_inductor/codegen/wrapper.py and torch/_inductor/codegen/triton.py where we currently just print the FX node name and the tensor (but a more useful integration will be coming later.)
I also introduce a debugging tool `debug_ir_traceback` which tracks tracebacks of where IRNodes were allocated, to help you understand why a node doesn't have an `origin_node`.
Signed-off-by: Edward Z. Yang <ezyang@meta.com>
Pull Request resolved: https://github.com/pytorch/pytorch/pull/100110
Approved by: https://github.com/voznesenskym
