Example 3 (XOR) / mean_square_error_loss issue
Regression due to laser migration, we still use `data` here:

https://github.com/mratsim/Arraymancer/blob/93200f27c19821232843a8b007669028fa8db298/src/arraymancer/nn/loss/mean_square_error_loss.nim#L23-L27

Instead we should use
`let grad = gradient.unsafe_raw_offset[0]`