In the old exporter we allow users to define a symbolic() method to bypass JIT tracing for a block of logic. We can allow users to do similar things by creating symbolic ops at export.
This PR implements `torch.onnx.ops.symbolic` and `torch.onnx.ops.symbolic_multi_out` to allow users to create onnx nodes symbolically with pt2 & fx. The custom pytorch ops were designed such that the attributes are encoded to be part of a valid fx op. Users provide shape and dtype for the meta function to produce the currect fake tensor during export.
An example is
Pull Request resolved: https://github.com/pytorch/pytorch/pull/148905
Approved by: https://github.com/titaiwangms
