Add default implementations of `assume` and `dot_assume`

[penelopeysm]

cf. TuringLang/Turing.jl#2360 (comment)

dot_assume's fallback method is here:

DynamicPPL.jl/src/context_implementations.jl

Lines 489 to 493 in 7f91c07

	function dot_assume(rng, spl::Sampler, ::Any, ::AbstractArray{<:VarName}, ::Any, ::Any)
	return error(
	"[DynamicPPL] $(alg_str(spl)) doesn't support vectorizing assume statement"
	)
	end

As far as I can tell assume doesn't have a fallback method so that would just need to be added.

[torfjelde]

IMO we should do this for all the methods.

Most samplers which are currently overloading the tilde-pipeline does so only to call updategid! and then nothing else (compared to SampleFromPrior).

These implementations subsequently go out of date (sometimes silently), making it all very annoying to maintain.

The only "objection" to this change is that we should (hopefully soon) remove the need for upategids!, in which case this default overload is not that useful.

[penelopeysm]

Probably no longer needed since SampleFromPrior etc. are being removed.