Improve onnxifi backend init time (#17375)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/17375
Previously we create the onnxGraph first and take it to the onnx manager for registration. It doesn't work well in practice. This diff takes "bring your own constructor" approach to reduce the resource spent doing backend compilation.
Reviewed By: kimishpatel, rdzhabarov
Differential Revision: D14173793
fbshipit-source-id: cbc4fe99fc522f017466b2fce88ffc67ae6757cf
