ds.typed_variables no longer works when subsetting a dataset

[clessig]

Describe the bug
A clear and concise description of what the bug is.

ds.typed_variables no longer works when subsetting a dataset

** Version number **
I am using the following versions/branch/sha1 of the anemoi packages
(alternatively the output of pip freeze)

anemoi-datasets==0.5.15
anemoi-transform==0.1.1
anemoi-utils==0.4.11

To Reproduce
Steps to reproduce the behavior:

1. Go to '...'
2. Run this '....'
3. See error

filename = 'aifs-ea-an-oper-0001-mars-o96-1979-2022-6h-v6.zarr'
ds = open_dataset( filename)
dss = open_dataset( ds, frequency='6h', start='1980', end='2020')
tv = ds.typed_variables()

Traceback (most recent call last):
File "", line 1, in
File ".venv/lib/python3.12/site-packages/anemoi/datasets/data/dataset.py", line 299, in typed_variables
constants = self.constant_fields
^^^^^^^^^^^^^^^^^^^^
AttributeError: 'Subset' object has no attribute 'constant_fields'

URL to sample input data
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Expected behavior
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subsetted ds should behave like those without it

Screenshots
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Additional context
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