Linearization/flattening of SimpleVarInfo (TuringLang#417)

This PR introduces a couple of things, though these are related:
1. `unflatten(original[, spl], x)`: converts from a certain input `x`, usually a `Vector`, into an instance similar to `original`.
   - Effectively the same as the current constructor `VarInfo(varinfo_old, spl, x)`.
   - I looked into using ParameterHandling.jl for this but decided against it for a couple of reasons:
     - Seems overkill.
     - `unflatten`-equivalent is constructed as a closure, which means that we need to keep track of this returned method rather than just using a "template" `AbstractVarInfo` + construction of unflattening requires construction of the flatten representation + the way one specifies the types is a bit too opinionated (which causes some issues with certain AD-frameworks) + closures can have less desirable performance characteristics.
     - The current Turing.jl-codebase is easily adapted to this `unflatten` since it's really just a matter of replacing calls `VarInfo(varinfo_old, spl, x)` with `unflatten(varinfo_old, spl, x)`. A ParameterHandling.jl approach will require more work.
2. `link!!` and `invlink!!`, BangBang-versions of `link!` and `invlink!`, respectively, with some differences:
   - These take additional arguments which should always be sufficient to determine the transformation. These are:
     - `model`
     - `sampler`
     - `t::AbstractTransformation`. This sets us up for allowing alternative transformations to be used. As of right now, this only has an affect when calling `link!!` and `invlink!!`, _not_ when used inside of the tilde-pipeline.
   - Also adds the logabsdet-jacobian term to the `logp`, so that `getlogp(vi) ≠ getlogp(link!!(vi))` holds. This allows us to compute, say, `logjoint` by _first_ linking `vi` in a single pass, and then compute `logjoint(settrans!(vi, NoTransformation()), θ_constrained)`. Such a pattern, in particular if the transformation has been specified by the user themselves, will usually have much better performance than the `logpdf_with_trans(..., true)` within the tilde-callstack.
3. `make_default_varinfo(rng, model, sampler)` which allows one to overload on a, say, per-model or model-sampler-combination basis to specify which implementation of `AbstractVarInfo` to use.
   - Not entirely happy with this approach 😕
   

EDIT: This should be merged _after_ TuringLang#420 

Co-authored-by: Hong Ge <[email protected]>

Project.toml

@@ -1,6 +1,6 @@
 name = "DynamicPPL"
 uuid = "366bfd00-2699-11ea-058f-f148b4cae6d8"
-version = "0.20.2"
+version = "0.21.0"
 
 [deps]
 AbstractMCMC = "80f14c24-f653-4e6a-9b94-39d6b0f70001"

docs/make.jl

@@ -3,7 +3,9 @@ using DynamicPPL
 using DynamicPPL: AbstractPPL
 
 # Doctest setup
-DocMeta.setdocmeta!(DynamicPPL, :DocTestSetup, :(using DynamicPPL); recursive=true)
+DocMeta.setdocmeta!(
+    DynamicPPL, :DocTestSetup, :(using DynamicPPL, Distributions); recursive=true
+)
 
 makedocs(;
     sitename="DynamicPPL",

docs/src/api.md

@@ -156,23 +156,56 @@ AbstractVarInfo
 
 ### Common API
 
+#### Accumulation of log-probabilities
+
 ```@docs
 getlogp
 setlogp!!
 acclogp!!
 resetlogp!!
 ```
 
+#### Variables and their realizations
+
 ```@docs
+keys
 getindex
+DynamicPPL.getindex_raw
 push!!
 empty!!
+isempty
 ```
 
 ```@docs
 values_as
 ```
 
+#### Transformations
+
+```@docs
+DynamicPPL.AbstractTransformation
+DynamicPPL.NoTransformation
+DynamicPPL.DynamicTransformation
+DynamicPPL.StaticTransformation
+```
+
+```@docs
+DynamicPPL.istrans
+DynamicPPL.settrans!!
+DynamicPPL.transformation
+DynamicPPL.link!!
+DynamicPPL.invlink!!
+DynamicPPL.default_transformation
+DynamicPPL.maybe_invlink_before_eval!!
+``` 
+
+#### Utils
+
+```@docs
+DynamicPPL.unflatten
+DynamicPPL.tonamedtuple
+```
+
 #### `SimpleVarInfo`
 
 ```@docs
@@ -191,10 +224,8 @@ TypedVarInfo
 One main characteristic of [`VarInfo`](@ref) is that samples are stored in a linearized form.
 
 ```@docs
-tonamedtuple
 link!
 invlink!
-istrans
 ```
 
 ```@docs

src/DynamicPPL.jl

@@ -59,7 +59,9 @@ export AbstractVarInfo,
     setorder!,
     istrans,
     link!,
+    link!!,
     invlink!,
+    invlink!!,
     tonamedtuple,
     values_as,
     # VarName (reexport from AbstractPPL)
@@ -126,27 +128,33 @@ export loglikelihood
 # Used here and overloaded in Turing
 function getspace end
 
-# Necessary forward declarations
 """
     AbstractVarInfo
 
 Abstract supertype for data structures that capture random variables when executing a
 probabilistic model and accumulate log densities such as the log likelihood or the
 log joint probability of the model.
 
-See also: [`VarInfo`](@ref)
+See also: [`VarInfo`](@ref), [`SimpleVarInfo`](@ref).
 """
 abstract type AbstractVarInfo <: AbstractModelTrace end
 
+const LEGACY_WARNING = """
+!!! warning
+    This method is considered legacy, and is likely to be deprecated in the future.
+"""
+
+# Necessary forward declarations
 include("utils.jl")
 include("selector.jl")
 include("model.jl")
 include("sampler.jl")
 include("varname.jl")
 include("distribution_wrappers.jl")
 include("contexts.jl")
-include("varinfo.jl")
+include("abstract_varinfo.jl")
 include("threadsafe.jl")
+include("varinfo.jl")
 include("simple_varinfo.jl")
 include("context_implementations.jl")
 include("compiler.jl")
@@ -155,5 +163,6 @@ include("compat/ad.jl")
 include("loglikelihoods.jl")
 include("submodel_macro.jl")
 include("test_utils.jl")
+include("transforming.jl")
 
 end # module