broadcast assign an array to ArrayPartition

```julia
import RecursiveArrayTools: ArrayPartition

a = ArrayPartition([1], [2])

# works
a[1] = 10
a[2] = 20

# does not
a .= [10, 20]
```


with `DimensionMismatch("destination axes (Base.OneTo(1),) are not compatible with source axes (Base.OneTo(2),)")` 