`groupby().max()` with Flox: `dtype` keyword not working.

What happened?

I'm applying a reduction on some data, but the dtype keyword documented here doesn't seem to work, without also specifying a fill_value.

Not sure if it's a bug, but otherwise it would be nice to make a note of this in the documentation.

What did you expect to happen?

I expect the output to have the dtype specified by the dtype keyword.

Minimal Complete Verifiable Example

Given a dataset with int16 data:

ds = xr.Dataset(
          {"test": (["x", "y"], np.array([[1,2,3], [4,5,6], [7,8,9]], dtype = "int16"))}, 
          coords = {"idx": ("x", [1,2,1])})

The following casts the "test" variable to float64 (as expected):

ds.groupby("idx").max(method = "blockwise", engine = "flox")

However, doing this:

ds.groupby("idx").max(method = "blockwise", engine = "flox", dtype = "int16")

Also gives float64 as output dtype.

Only once I also specify a fill_value, the output follows the specified dtype, e.g.:

x = ds.groupby("idx").max(method = "blockwise", engine = "flox", dtype = "int16", fill_value = 99)

gives x["test"].dtype.name == "int16", or:

x = ds.groupby("idx").max(method = "blockwise", engine = "flox", dtype = "int32", fill_value = 99)

gives x["test"].dtype.name == "int32"

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: 3.12.2 | packaged by conda-forge | (main, Feb 16 2024, 20:54:21) [Clang 16.0.6 ] python-bits: 64 OS: Darwin OS-release: 23.5.0 machine: arm64 processor: arm byteorder: little LC_ALL: None LANG: None LOCALE: (None, 'UTF-8') libhdf5: 1.14.3 libnetcdf: 4.9.2 xarray: 2024.2.0 pandas: 2.2.1 numpy: 1.26.4 scipy: 1.12.0 netCDF4: 1.6.5 pydap: None h5netcdf: None h5py: None Nio: None zarr: None cftime: 1.6.3 nc_time_axis: None iris: None bottleneck: 1.3.8 dask: 2024.6.2 distributed: 2024.6.2 matplotlib: 3.8.3 cartopy: None seaborn: None numbagg: 0.8.1 fsspec: 2024.2.0 cupy: None pint: None sparse: None flox: 0.9.10 numpy_groupies: 0.11.2 setuptools: 70.2.0 pip: 24.1.2 conda: None pytest: 8.0.2 mypy: None IPython: 8.22.1 sphinx: 5.3.0

pydata / xarray