let `x=[1 2; 3 4]`

and `y=copy(x)`

I want to stack the two arrays together on the 4th dimension.

E.g. `xy = cat(x, y, dims=4)`

will get what I wanted

However the above is fine but when you apply the same technique to a large data such as the MNIST dataset the speed is really slow e.g.

```
using Flux, Flux.Data.MNIST
imgs = MNIST.images()
X = cat(float.(imgs)..., dims = 4) # SLOW!!!
```

What’s the most efficient code to create `X`

? (which is done by stacking 60,000 28x28 matrices together on the 4th dimension.)