Consider omitting unchunked dimensions from Key objects created with DatasetToChunks

Currently we have (from https://xarray-beam.readthedocs.io/en/latest/read-write.html):
```python
with beam.Pipeline() as p:
p | xbeam.DatasetToChunks(ds, chunks={'time': 1000}) | beam.MapTuple(print_summary)
```
```
Key(offsets={'lat': 0, 'lon': 0, 'time': 0}, vars=None)
with
Key(offsets={'lat': 0, 'lon': 0, 'time': 1000}, vars=None)
with
Key(offsets={'lat': 0, 'lon': 0, 'time': 2000}, vars=None)
with
```

Should we instead omit `lat` and `lon` from these keys? This is less explicit but also more flexible, e.g,. if replacing these dimensions entirely with different dimensions, you don't need to update the keys.