Avoid use of collect

### Description

In multiple situations, I am using `CuArray(collect( ... ) )` because `AbstractFFTs.fft` returns an `Array` and not a `CuArray` when the input is a `SubArray` with `CuArray` parent. This breaks the code.

The solution using collect+casting allocates GPU memory before performing fft and fixes the return type at the cost of memory allocation. 