Zip and azip! behavior related to inputted NdProducers

Start by compiling the two azip! examples from the issue and compare the inferred dimensions of lanes(Axis(0)) with axis_iter(Axis(0)). Then inspect Zip, azip!, and IntoNdProducer behavior to determine whether the mismatch is an implementation bug or an intentional constraint; done means establishing and documenting the expected behavior, with a regression test if a bug is confirmed.

Based on trying to help in Issue #452 , I found some interesting behavior regarding either Zip or azip!. I've given examples down below

    let a = arr3(&[[[ 0,  1,  2],
                [ 3,  4,  5]],
               [[ 6,  7,  8],
                [ 9, 10, 11]]]);
    let inner0 = a.lanes(Axis(0));

    let trial = arr1(&[0,0,0,0,0,0]);
    let tr_iter = trial.axis_iter(Axis(0));

    //The below will error out when compiling.
    azip!(ref output (inner0), ref trial (tr_iter) in {println!("{:?}, {:?}",output, trial); });

The compiler returns the following error:

error[E0271]: type mismatch resolving `<ndarray::iter::AxisIter<'_, {integer}, ndarray::Dim<[usize; 0]>> as ndarray::IntoNdProducer>::Dim == ndarray::Dim<[usize; 2]>`
   --> src/main.rs:141:5
    |
141 |     azip!(ref output (inner0), ref trial (tr_iter) in {println!("{:?}, {:?}",output, trial); });
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected an array with a fixed size of 1 elements, found one with 2 elements
    |
    = note: expected type `ndarray::Dim<[usize; 1]>`
               found type `ndarray::Dim<[usize; 2]>`
    = note: this error originates in a macro outside of the current crate

error[E0599]: no method named `apply` found for type `ndarray::Zip<(ndarray::iter::Lanes<'_, {integer}, ndarray::Dim<[usize; 2]>>, ndarray::iter::AxisIter<'_, {integer}, ndarray::Dim<[usize; 0]>>), ndarray::Dim<[usize; 2]>>` in the current scope
   --> src/main.rs:141:5
    |
141 |     azip!(ref output (inner0), ref trial (tr_iter) in {println!("{:?}, {:?}",output, trial); });
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: this error originates in a macro outside of the current crate

However, if I were to roll what inner0 outputs directly, I can get it to work no problem as seen below:

    let trial = arr1(&[0,0,0,0,0,0]);
    let tr_iter = trial.axis_iter(Axis(0));

    let b = arr2(&[[0,1,2,3,4,5],[6,7,8,9,10,11]]);
    let b_iter = b.axis_iter(Axis(1));
    //The compiler is perfectly okay with this code.
    azip!(ref output (b_iter), ref trial (tr_iter) in {println!("{:?}, {:?}",output, trial); });

I feel that either method should work, since we contain the same number of "items" to iterate over in each version. So, I was curious if this is simply an error/bug in the code or is there a design choice behind one method failing to compile and the other one working.