[Executorch][to_backend] Introduce preprocess_multimethod (#9823)

### summary

We introduce a new preprocess_multimethod. This takes the place of
preprocess all in the following discussion:
https://github.com/pytorch/executorch/discussions/8230

This is a class method so that we can allow the default implementation
to leverage the static preprocess method. The default implementation
just applies preprocess on all the exported programs given.

### Testing
Testing done in follow up PR 
https://github.com/pytorch/executorch/pull/9824