id,node_id,number,title,user,state,locked,assignee,milestone,comments,created_at,updated_at,closed_at,author_association,active_lock_reason,draft,pull_request,body,reactions,performed_via_github_app,state_reason,repo,type
809366777,MDExOlB1bGxSZXF1ZXN0NTc0MjQxMTE3,4915,Better rolling reductions,2448579,closed,0,,,6,2021-02-16T14:37:49Z,2023-04-13T15:46:18Z,2021-02-19T19:44:04Z,MEMBER,,0,pydata/xarray/pulls/4915,"<!-- Feel free to remove check-list items aren't relevant to your change -->

Implements most of https://github.com/pydata/xarray/issues/4325#issuecomment-716399575
``` python
%load_ext memory_profiler

import numpy as np
import xarray as xr

temp = xr.DataArray(np.zeros((5000, 500)), dims=(""x"", ""y""))

roll = temp.rolling(x=10, y=20)

%memit roll.sum()
%memit roll.reduce(np.sum)
%memit roll.reduce(np.nansum)  # master  branch behaviour
```
```
peak memory: 245.18 MiB, increment: 81.92 MiB
peak memory: 226.09 MiB, increment: 62.69 MiB
peak memory: 4493.82 MiB, increment: 4330.43 MiB
```
- [x] xref #4325
- [x] asv benchmarks added
- [x] Passes `pre-commit run --all-files`
- [x] User visible changes (including notable bug fixes) are documented in `whats-new.rst`
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