JIT: Eliminate SumToSize by using Optional Lists (#18697)
Summary:
This PR is a eliminates unneeded grad_sum_to_size and in particular speeds up the LSTM backward by allowing better fusion.
It consists of two parts:
- In AutoDiff, record broadcasting sizes only if the broadcast output size is different from the input size, otherwise record None.
- The specialization of Optional arguments (#18407) allows us to then eliminate ` _grad_sum_to_size(t, None)` in the peephole optimization step.
Thus, in the LSTM case, no SumToSize remain in the crucial fusion group. The trick here is that we can specialize on the runtime information from the forward.
I'm testing that different broadcasting situations lead to different graphs.
I didn't move all symbolic_script _grad_sum_to_size to the new logic, but it might be better to do this incrementally, anyway.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18697
Differential Revision: D15482076
Pulled By: wanchaol
fbshipit-source-id: 7f89367e35b8729910077c95c02bccefc8678afb
