`find_MAP` is failing in `V4`

[ricardoV94]

While working on #4770 I found that find_MAP is failing quite often after the V4 refactoring.

Most issues seem related to gradients of discrete vars and/or models with both discrete and continuous unobserved variables. The failing tests in here are marked with an xfail for the time being.

[michaelosthege]

Could it be a problem with the initialization too? (crossref #4752)
I found "find_MAP" to be quite brittle when the initialization is bad. Not only because optimization is hard, but also because the automatic convergence detection often makes it give up after a handful of iterations.

[ricardoV94]

I have the impression it's more than that. But it was some time since I had a look