Lazy SUM with 1D weights sometimes fails

Start at iris/analysis/init.py, especially lazy_aggregate and _lazy_sum, then run the provided Dask-backed reproduction. Done means collapsing the cube with iris.analysis.SUM and 1D weights completes without the broadcasting error and preserves the expected result.

🐛 Bug Report

Collapsing a cube with lazy data using iris.analysis.SUM and 1D weights

cube = cube.collapsed('a', iris.analysis.SUM, weights=[0, 1]) 

sometimes fails due to broadcasting errors:

Traceback (most recent call last):
  File "/home/b/b309141/scripts/iris/cube_weights.py", line 24, in <module>
    cube = cube.collapsed('a', iris.analysis.SUM, weights=[0, 1])
  File "/work/bd0854/b309141/mambaforge/envs/esm/lib/python3.10/site-packages/iris/cube.py", line 3785, in collapsed
    data_result = aggregator.lazy_aggregate(
  File "/work/bd0854/b309141/mambaforge/envs/esm/lib/python3.10/site-packages/iris/analysis/__init__.py", line 545, in lazy_aggregate
    return self.lazy_func(data, axis=axis, **kwargs)
  File "/work/bd0854/b309141/mambaforge/envs/esm/lib/python3.10/site-packages/iris/analysis/__init__.py", line 1136, in inner_stat
    dask_result = dask_stats_function(array, axis=axis, **kwargs)
  File "/work/bd0854/b309141/mambaforge/envs/esm/lib/python3.10/site-packages/iris/analysis/__init__.py", line 1136, in inner_stat
    dask_result = dask_stats_function(array, axis=axis, **kwargs)
  File "/work/bd0854/b309141/mambaforge/envs/esm/lib/python3.10/site-packages/iris/analysis/__init__.py", line 1408, in _lazy_sum
    wsum = da.sum(weights_in * array, **kwargs)
  File "/work/bd0854/b309141/mambaforge/envs/esm/lib/python3.10/site-packages/dask/array/core.py", line 223, in wrapper
    return f(self, other)
  File "/work/bd0854/b309141/mambaforge/envs/esm/lib/python3.10/site-packages/dask/array/core.py", line 2349, in __rmul__
    return elemwise(operator.mul, other, self)
  File "/work/bd0854/b309141/mambaforge/envs/esm/lib/python3.10/site-packages/dask/array/core.py", line 4762, in elemwise
    broadcast_shapes(*shapes)
  File "/work/bd0854/b309141/mambaforge/envs/esm/lib/python3.10/site-packages/dask/array/core.py", line 4690, in broadcast_shapes
    raise ValueError(
ValueError: operands could not be broadcast together with shapes (2,) (2, 3)

How To Reproduce

import dask.array as da
import iris
print("iris version:", iris.__version__)
print()
from iris.coords import AuxCoord
from iris.cube import Cube
aux_coord = AuxCoord([1, 2], var_name='a')
cube = Cube(da.arange(2 * 3).reshape(2, 3), aux_coords_and_dims=[(aux_coord, 0)])
cube = cube.collapsed('a', iris.analysis.SUM, weights=[0, 1])

Expected behaviour

No fail.

Environment

• OS & Version: Red Hat Enterprise Linux 8.5 (Ootpa)
• Iris Version: Tested with 3.2.1 and 3.5.0.dev3, so I guess versions in between are also affected