Fix use of ONNX optimizer by Caffe2 backend
Fixes https://github.com/pytorch/pytorch/issues/69674
The fix is Back Compatible with any Caffe2 build. It simply tries to use `onnxptimizer` module when `onnx.optimizer` is not available.
`onnx.optimizer` does not exist since ONNX 1.9 (April 2021) as the code was moved to a different [repo](https://github.com/onnx/onnxoptimizer)
If both `onnx<1.9` and `onnxoptimizer` are not found, the current fallback behavior is maintained (no ONNX optimization happens). Otherwise, the ONNX optimization pass will run from whatever module it is found.
This PR does not require or enforce a direct package dependency to work
Pull Request resolved: https://github.com/pytorch/pytorch/pull/75718
Approved by: https://github.com/BowenBao, https://github.com/malfet
