FR: Add `Mish` as activation function

[Tato14]

Since it seems that Mish activation works nicely in Segmentation tasks. I would like to add it to the learner as performed here. I will take a look how this is done but just in case its an easy point that I am not aware I tagged it as FR.
Thanks!

[WaterKnight1998]

Since it seems that Mish activation works nicely in Segmentation tasks. I would like to add it to the learner as performed here. I will take a look how this is done but just in case its an easy point that I am not aware I tagged it as FR.
Thanks!

Hi,

In my other project (Deep-Tumour-Spheroid) where I used this library to segment brain tumours, I obtained worse results with Mish than with default activation layer.

As you know in Machine Learning there is no a master parameter or option that works well with all the problems. So you must tune the parameters to each specific problem!