penzai named array axis

• pz.nx.NamedArray class wraps an ordinary array, and assigns each axis to either a position or a name (but not both)
• two shapes:
  • .positional_shape attribute is the sequence of dimension sizes for the positional axes in the array
  • .named_shape attribute is a dictionary mapping axis names to their dimension sizes

pz.nx.wrap(jnp.ones([3, 4, 5])).tag("foo", "bar", "baz")
my_named_array.untag("bar", "foo", "baz").unwrap()

• operations on NamedArrays always act only on the positional axes
  • untag, move those axes to the .positional_shape
  • operate, calling pz.nx.nmap(some_positional_op)(...args...)
  • retag, .tag to move the resulting axes from the .positional_shape back to the .named_shape

def named_dot(x: pz.nx.NamedArray, features_axis: str) -> pz.nx.NamedArray:
	pos_x = x.untag(features_axis)
	pos_kernel = kernel.value.untag("out_features", "in_features")
	pos_y = pz.nx.nmap(jnp.dot)(pos_kernel, pos_x)
	return pos_y.tag(features_axis)