Use pax_fiddle.sub_field for FiddleBaseLayer.params_init.
Use pax_fiddle.sub_field for FiddleBaseLayer.params_init.

This ensures that `params_init` will be deeply configurable.

Also add @auto_config to the WeightInit factory staticmethods.  This ensures that when they are configured using Fiddle, the configuration parameters will be `method` and `scale` (and not the parameters to the factory method).

PiperOrigin-RevId: 485484876

google/praxis,google/praxis