BUG: `pz.chk.check_structure` erroneously requires all named array axes to be strings.

[amifalk]

import jax
from penzai import pz

my_layer = pz.nn.Linear.from_config(
    "my_layer", jax.random.key(0), input_axes={"in_axis": 2}, output_axes={"out_axis": 3}
)

nonstr_batch_axis = pz.nx.TmpPosAxisMarker()

my_layer(pz.nx.zeros({"in_axis": 2, nonstr_batch_axis: 3}))

The following raises an assertion error here:

penzai/penzai/core/shapecheck.py

Line 621 in aac7808

assert isinstance(subkey, str)

I don't think any assumptions of shapecheck require this, and it limits the ability to do automatic axis naming without worry of axis name overlaps.