Observe vectorisation issue.

[trappmartin]

We increase vi.num_produce on observe only for a single distribution and not during vectorisation.
Note: This is only required for Particle based samplers.

Turing.jl/src/inference/Inference.jl

Lines 182 to 187 in 65371ce

	function observe(spl::A,
	dist::Distribution,
	value::Any,
	vi::VarInfo) where {A<:Union{SampleFromPrior, SampleFromUniform}}
	vi.num_produce += one(vi.num_produce)

[mohdibntarek]

Closed via #965.