[Finalize API] Samplers

Created by: PhilipVinc

Right now our Sampler API states that all samplers should define sample_chain and sample_step. This is a bit weird, as sample_step is really a MCMC-specific topic, which should not be defined for things like direct samplers (ARNN, Exact...). In fact, ARNNSampler does not define it, which is the correct thing to do.

Also, this ends up having n_chains for ARNNSampler and Exactampler which makes no sense at all.

I think we should slightly modify the current API: AbstractSampler should only define sample or sample_batch and return samples as a 2 dimensional vector. MCSamplers should also defined sample_step and return samples as a 3 dimentional tensor, as they do now.

The only weird thing is that we would have that vs.samples will now be either a 2D or 3D vector, but as all our code works with both cases right now, I don't think this is particularly an issue.

@wdphy16 I'm assigning this to you. After everyone has agreed on this change, could you work on that when you have some time? it should be relatively straightforward and fast to implement.