What happened?

I'm getting task graphs > 1GB, I think possibly because the full indexes are being included in every task?

What did you expect to happen?

Only the relevant sections of the index would be included

Minimal Complete Verifiable Example

```Python da = xr.tutorial.load_dataset('air_temperature')

Dropping the index doesn't generally matter that much...

len(cloudpickle.dumps(da.chunk(lat=1, lon=1)))

15569320

len(cloudpickle.dumps(da.chunk().drop_vars(da.indexes)))

15477313

But with .map_blocks, it really matters — it's really big with the indexes, and the same size without:

len(cloudpickle.dumps(da.chunk(lat=1, lon=1).map_blocks(lambda x: x)))

79307120

len(cloudpickle.dumps(da.chunk(lat=1, lon=1).drop_vars(da.indexes).map_blocks(lambda x: x)))

16016173

```

MVCE confirmation

• [X] Minimal example — the example is as focused as reasonably possible to demonstrate the underlying issue in xarray.
• [X] Complete example — the example is self-contained, including all data and the text of any traceback.
• [X] Verifiable example — the example copy & pastes into an IPython prompt or Binder notebook, returning the result.
• [X] New issue — a search of GitHub Issues suggests this is not a duplicate.
• [X] Recent environment — the issue occurs with the latest version of xarray and its dependencies.

Relevant log output

No response

Anything else we need to know?

No response

Environment

INSTALLED VERSIONS ------------------ commit: None python: 3.9.18 (main, Aug 24 2023, 21:19:58) [Clang 14.0.3 (clang-1403.0.22.14.1)] python-bits: 64 OS: Darwin OS-release: 22.6.0 machine: arm64 processor: arm byteorder: little LC_ALL: en_US.UTF-8 LANG: None LOCALE: ('en_US', 'UTF-8') libhdf5: 1.12.2 libnetcdf: None xarray: 2023.10.1 pandas: 2.1.1 numpy: 1.26.1 scipy: 1.11.1 netCDF4: None pydap: None h5netcdf: 1.1.0 h5py: 3.8.0 Nio: None zarr: 2.16.0 cftime: 1.6.2 nc_time_axis: None PseudoNetCDF: None iris: None bottleneck: 1.3.7 dask: 2023.5.0 distributed: 2023.5.0 matplotlib: 3.6.0 cartopy: None seaborn: 0.12.2 numbagg: 0.6.0 fsspec: 2022.8.2 cupy: None pint: 0.22 sparse: 0.14.0 flox: 0.7.2 numpy_groupies: 0.9.22 setuptools: 68.1.2 pip: 23.2.1 conda: None pytest: 7.4.0 mypy: 1.6.1 IPython: 8.14.0 sphinx: 5.2.1