Rename sharding_names to out_sharding in NNX Variable metadata
This CL renames the sharding_names attribute to out_sharding for better
consistency with the sharding API. The new name more clearly indicates
the purpose of this metadata field.
## Changes
- Bump Flax version to 0.12.4
- Core changes in variablelib.py:
- Add sharding_names to out_sharding metadata remapping for backward compatibility
- Add deprecated sharding_names property that returns out_sharding with a warning
- Update nnx/spmd.py, core/spmd.py, core/meta.py, linen/spmd.py to use out_sharding
- Update all NNX tests to use the new attribute name
- Update qwix flax_util.py to check for out_sharding first, with fallback to sharding_names
- Update maxtext initializers.py to check for out_sharding first
- Update documentation and examples to use out_sharding
## Backward Compatibility
Existing code using sharding_names will continue to work via:
- Metadata remapping during Variable creation
- Deprecated Variable.sharding_names property
PiperOrigin-RevId: 869269899
