LOG_COSH

Computes the logarithm of the hyperbolic cosine of the prediction error.

``log(cosh(x))`` is approximately equal to ``(x ** 2) / 2`` for small ``x`` and to ``abs(x) - log(2)`` for large ``x``. This means that logcosh works mostly like the mean squared error, but will not be so strongly affected by the occasional wildly incorrect prediction.