ParallelRange with Sliced Matrix

If you slice a matrix in the ParallelRange matmul does not detect the change in dimension and fails:

```
res = af.constant(0,20,20,1000 )
left = af.constant(0, 20, 40)
right = af.constant(0, 40,20, 1000)

af.matmul(left, right[:,:,0]) # this works
for ii in af.ParallelRange(10):
res[:,:,ii] = af.matmul(left, right[:,:,ii]) #will fail
```

Start by running the provided ParallelRange reproducer with the sliced right[:,:,ii] input and compare it with the standalone matmul that succeeds. Trace the ParallelRange and matmul entry points to find where the slice dimensions are handled; done means the looped matmul completes without failure and produces the expected result shape.

Written by the indexing model from the issue text.