[ONNX] Support complex in FX exporter (#100554)
Previous to the PR, the complex dtype would only fail. This PR keeps torch.fx.Graph with complex dtype, while mapping them to float dtype in torchscript(onnx) graph with real representation.
The change happens in multiple files:
1. `placeholder`: Apply torch.view_as_real() before sending fake tensor to graph building.
2. `call_function`: Fill in TorchScriptTensor dtype and shape with real representation dtype and shape.
3. Registry: Add `is_complex`, and supports complex onnxfunction.
4. Dispatcher: Filter with/out complex onnxfunction before opschema matching, based on the dtype in torch args
5. Test cases: input/output view_as_real for result comparisons.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/100554
Approved by: https://github.com/BowenBao
