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
1072473598,I_kwDOAMm_X84_7KX-,6051,"Check for just ... in stack etc, and raise with a useful error message",2448579,closed,0,,,4,2021-12-06T18:35:27Z,2022-01-03T23:05:23Z,2022-01-03T23:05:23Z,MEMBER,,,,"<!-- Please do a quick search of existing issues to make sure that this has not been asked before. -->

**Is your feature request related to a problem? Please describe.**

The following doesn't work
``` python
import xarray as xr

da = xr.DataArray([[1,2],[1,2]], dims=(""x"", ""y""))
da.stack(flat=...)
```

**Describe the solution you'd like**
This could be equivalent to
``` python
da.stack(flat=da.dims)
```

I *think* using `ds.dims` it should be fine for datasets too.

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