Numerical grad inconsistencies: grad * op_backward or directly op_backward
Convolution:
https://github.com/mratsim/Arraymancer/blob/69f3c32b7a375092bba869100aea23c48d06232e/tests/nn_primitives/test_nnp_convolution.nim#L153-L158

Softmax cross-entropy:
https://github.com/mratsim/Arraymancer/blob/69f3c32b7a375092bba869100aea23c48d06232e/tests/nn_primitives/test_nnp_loss.nim#L39-L47

Not sure if it's normal or not.