Some basic automatic signature support for SavedModel export

Adds some checking for missing signatures (i.e. it will no longer let you export nothing). But if there's only one decorated method it'll try exporting that.

The hope is that we can follow up with support for exporting Keras Models (at least functional models) without requiring an explicit defun. But for now we'll at least have basic sanity checking.

PiperOrigin-RevId: 219869746