Series.std() fails when using extension dtypes

[jrbourbeau]

Series.std() fails if I use nullable dtypes (e.g. Int64) or pyarrow-backed dtypes (e.g. int64[pyarrow])

import pandas as pd
import dask.dataframe as dd

# pandas
s = pd.Series([1, 2, 3, 4], dtype="Int64")
print(f"{s.std() = }")

# Dask
ds = dd.from_pandas(s, npartitions=3)
print(f"{ds.std() = }")

fails with

Traceback (most recent call last):
  File "/Users/james/projects/dask/dask/dask/dataframe/dispatch.py", line 128, in make_meta
    return make_meta_dispatch(x, index=index)
  File "/Users/james/projects/dask/dask/dask/utils.py", line 639, in __call__
    meth = self.dispatch(type(arg))
  File "/Users/james/projects/dask/dask/dask/utils.py", line 633, in dispatch
    raise TypeError(f"No dispatch for {cls}")
TypeError: No dispatch for <class 'pandas._libs.missing.NAType'>

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/Users/james/projects/dask/dask/test-nullable-std.py", line 10, in <module>
    print(f"{ds.std() = }")
  File "/Users/james/projects/dask/dask/dask/dataframe/core.py", line 117, in wrapper
    return func(self, *args, **kwargs)
  File "/Users/james/projects/dask/dask/dask/dataframe/core.py", line 2454, in std
    result = map_partitions(
  File "/Users/james/projects/dask/dask/dask/dataframe/core.py", line 6630, in map_partitions
    meta = _get_meta_map_partitions(args, dfs, func, kwargs, meta, parent_meta)
  File "/Users/james/projects/dask/dask/dask/dataframe/core.py", line 6744, in _get_meta_map_partitions
    meta = make_meta(meta, index=meta_index, parent_meta=parent_meta)
  File "/Users/james/projects/dask/dask/dask/dataframe/dispatch.py", line 132, in make_meta
    return func(x, index=index)
  File "/Users/james/projects/dask/dask/dask/dataframe/backends.py", line 287, in make_meta_object
    return _nonempty_scalar(x)
  File "/Users/james/projects/dask/dask/dask/dataframe/utils.py", line 340, in _nonempty_scalar
    raise TypeError(f"Can't handle meta of type '{typename(type(x))}'")
TypeError: Can't handle meta of type 'pandas._libs.missing.NAType'

If I switch to dtype="int64[pyarrow]" I get

Traceback (most recent call last):
  File "/Users/james/projects/dask/dask/test-nullable-std.py", line 10, in <module>
    print(f"{ds.std() = }")
  File "/Users/james/projects/dask/dask/dask/dataframe/core.py", line 117, in wrapper
    return func(self, *args, **kwargs)
  File "/Users/james/projects/dask/dask/dask/dataframe/core.py", line 2440, in std
    v = numeric_dd.var(skipna=skipna, ddof=ddof, split_every=split_every)
  File "/Users/james/projects/dask/dask/dask/dataframe/core.py", line 117, in wrapper
    return func(self, *args, **kwargs)
  File "/Users/james/projects/dask/dask/dask/dataframe/core.py", line 2272, in var
    result = self._var_1d(self, skipna, ddof, split_every)
  File "/Users/james/projects/dask/dask/dask/dataframe/core.py", line 2373, in _var_1d
    if not np.issubdtype(column.dtype, np.number):
  File "/Users/james/mambaforge/envs/dask-py39/lib/python3.9/site-packages/numpy/core/numerictypes.py", line 416, in issubdtype
    arg1 = dtype(arg1).type
TypeError: Cannot interpret 'int64[pyarrow]' as a data type

[jrbourbeau]

Just noting that on the current main, the Int64 example above now passes, but the int64[pyarrow] one still fails