Add dataset API for getting a fake batch from the dataset.
Add dataset API for getting a fake batch from the dataset.

There are instances where we need to know the (typical) shape of a batch output from the dataset, including inputs, targets, and any metadata, but we do not want to full instantiate the pipeline and grab data from disk/network. The most immediate example is in testing.

NOTES
- This first CL simply declares the API and sets `fake_batch_getter` to None for all datasets since there are no dependencies yet.
- As expected, tests pass.
- We intend to implement this function for all datasets (and any future datasets), but will prioritize those datasets for the upcoming MLCommons competition.

PiperOrigin-RevId: 483683289

google/init2winit