MNIST OpenMP accuracy 25% less than single threaded
Compiling the [MNIST example](https://github.com/mratsim/Arraymancer/blob/8a92d825ae436e868c82709ade7553a8a6716816/examples/ex02_handwritten_digits_recognition.nim) with OpenMP leads to much worse accuracy for no apparent reason.

After the first epoch
Random seed 1337: Single-thread: 76.2%, OpenMP 51%
Random seed 42: Single-thread: 83.2%, OpenMP 57.5%