issues: 319085244

I have a list of DataArrays with three dimensions. For each item in the list, two of the dimensions are a single value but the combination of all items would yield the full combinatorial values.

Code Sample

```python import itertools import numpy as np import xarray as xr

ds = []
for vals_dim1, vals_dim2 in itertools.product(list(range(2)), list(range(3))):
    d = xr.DataArray(np.random.rand(1, 1, 4),
                     coords={'dim1': [vals_dim1], 'dim2': [vals_dim2], 'dim3': range(4)},
                     dims=['dim1', 'dim2', 'dim3'])
    ds.append(d)

```

Expected Output

I then want to combine these complimentary DataArrays but none of what I tried so far seems to work. The result should be a DataArray with shape |2x3x4| and dimensions dim1: |2|, dim2: |3|, dim3: |4|.

The following do not work: ```python # does not automatically infer dimensions and fails with # "ValueError: conflicting sizes for dimension 'concat_dim': length 2 on 'concat_dim' and length 6 on <this-array>" ds = xr.concat(ds, dim=['dim1', 'dim2'])

# will still try to insert a new `concat_dim` and fails with
# "ValueError: conflicting MultiIndex level name(s): 'dim1' (concat_dim), (dim1) 'dim2' (concat_dim), (dim2)"
import pandas as pd
dims = [[0] * 3 + [1] * 3, list(range(3)) * 2]
dims = pd.MultiIndex.from_arrays(dims, names=['dim1', 'dim2'])
ds = xr.concat(ds, dim=dims)

# fails with
# AttributeError: 'DataArray' object has no attribute 'data_vars'
ds = xr.auto_combine(ds)

```

Output of xr.show_versions()