Example custom op with output type inferencing (#23916)
### Description
<!-- Describe your changes. -->
Add example of a custom op that is required to do type inference for the
output type for the model load to work.
Also acts as an example of how to override an ONNX op with a custom
implementation.
### Motivation and Context
<!-- - Why is this change required? What problem does it solve?
- If it fixes an open issue, please link to the issue here. -->
#23891
