[ONNX] Add silu operator support for onnx (#51193) (#51519)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/51519
Support for yolov5 compound-scaled object detection models export.
Test Plan: Imported from OSS
Reviewed By: pbelevich
Differential Revision: D26203120
Pulled By: SplitInfinity
fbshipit-source-id: c70bd730ee5d6f8bdebaf8ff764b94ffe7673808
