Disabling ONNX IR v4 sematics for opset 8 or lower. (#28990)
Summary:
Currently, `keep_initializers_as_input` argument in `torch.onnx.export` API can be used to choose whether to export an ONNX model with IR v3 or v4 semantics. Currently, the implementation does not check for which opset is being used for export. This is an issue because ONNX IR v4 is valid only for opset 9 and above (as listed [here](https://github.com/onnx/onnx/releases/tag/v1.4.0)), and opset 8 or lower export with `keep_initializers_as_input=False` will create a illegal ONNX graph.
This change fixes this by introducing a check on opset version when deciding whether to export ONNX IR v3 or v4.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/28990
Reviewed By: hl475
Differential Revision: D18352523
Pulled By: houseroad
fbshipit-source-id: 7e9055d405c3faf52b80a8de0d04186d4c350c15
