jit around pmap'ed FP & BP is a problem when pmap'ing SIR cost functions

[clemisch]

The jit around pmap might lead to performance issues when using pmap on SIR cost functions.

When a user only uses FP or BP on its own, the jit seems to be faster. But it leads to synchronizing all data on every device, which is not desirable if any substantial computation comes afterwards. This could be filtering projections for FBP or computing cost function on projections for SIR.

So it might be better to disable automatic jit of pmap. We could instead provide a function for FBP (combining filtering and BP) and jit around that?