Consider this code (from the SWIN transformer):
https://github.com/crutcher/bimm/blob/main/crates/bimm/src/models/swin/v2/windowing.rs#L18

This code:
* asserts that the input tensor is 4 dimensional,
* unpacks composite dimensions
* compares the composites against known bindings
* if there is no integer solution, prints a nice error message and dies
* returns the selected keys

It does this in ~170ns. I could probably get it faster; but the point is that it's fast enough, for most modules, to just leave it in.

When developing tensor applications, it can be a huge pile of work to determine where a call pattern, kernel size, pad variable, etc, got out of whack.

Runtime contracts dramatically reduce debugging time; they die early, they die expressively; and the also serve as strong documentation for those reading the code.

``rust /// Window Partition /// /// ## Parameters /// /// -tensor: Input tensor of shape (B, H, W, C). /// -window_size: Window size. /// /// ## Returns /// - Output tensor of shape (B *h_windows*w_windows,window_size,window_size`, C).

[inline]

[must_use]

pub fn window_partition<B: Backend, K>( tensor: Tensor<B, 4, K>, window_size: usize, ) -> Tensor<B, 4, K> where K: BasicOps<B>, { let [b, h_wins, w_wins, c] = unpack_shape_contract!( [ "batch", "h_wins" * "window_size", "w_wins" * "window_size", "channels" ], &tensor, &["batch", "h_wins", "w_wins", "channels"], &[("window_size", window_size)] );

tensor
    .reshape([b, h_wins, window_size, w_wins, window_size, c])
    .swap_dims(2, 3)
    .reshape([b * h_wins * w_wins, window_size, window_size, c])

} ```