Special-case ReshapeTransform for singleton inputs

[penelopeysm]

Closes #698.

I can confirm that this change fixes the two Turing tests that I listed here TuringLang/Turing.jl#2376 (comment). It might well fix all the other tests too, since they are giving the same error message.

using Turing
import ReverseDiff

alg = HMC(0.01, 5; adtype=AutoReverseDiff())

@model function vdemo6()
    x = Vector{Real}(undef, 10)
    @. x ~ InverseGamma(2, 3)
end
sample(vdemo6(), alg, 10)

@model function vdemo7()
    x = Array{Real}(undef, N, N)
    @. x ~ [InverseGamma(2, 3) for i in 1:N]
end
sample(vdemo7(), alg, 1000)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.23%. Comparing base (bd9f465) to head (8c9cc71).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #699   +/-   ##
=======================================
  Coverage   79.22%   79.23%           
=======================================
  Files          30       30           
  Lines        4212     4214    +2     
=======================================
+ Hits         3337     3339    +2     
  Misses        875      875           

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[coveralls]

Pull Request Test Coverage Report for Build 11510571276

Details

• 3 of 3 (100.0%) changed or added relevant lines in 1 file are covered.
• 93 unchanged lines in 18 files lost coverage.
• Overall coverage decreased (-1.5%) to 78.11%

---

Files with Coverage Reduction	New Missed Lines	%
src/model.jl	1	93.68%
src/sampler.jl	1	92.73%
ext/DynamicPPLEnzymeCoreExt.jl	1	0.0%
src/contexts.jl	3	75.76%
src/context_implementations.jl	3	63.67%
src/test_utils.jl	3	85.31%
src/model_utils.jl	3	19.64%
src/varnamedvector.jl	3	89.42%
src/distribution_wrappers.jl	4	44.12%
ext/DynamicPPLForwardDiffExt.jl	4	66.67%

Totals
Change from base Build 11501458234:	-1.5%
Covered Lines:	3290
Relevant Lines:	4212

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 11516553436

Details

• 3 of 3 (100.0%) changed or added relevant lines in 1 file are covered.
• 93 unchanged lines in 18 files lost coverage.
• Overall coverage increased (+0.01%) to 79.633%

---

Files with Coverage Reduction	New Missed Lines	%
src/model.jl	1	93.68%
src/sampler.jl	1	92.73%
ext/DynamicPPLEnzymeCoreExt.jl	1	0.0%
src/contexts.jl	3	75.76%
src/context_implementations.jl	3	63.67%
src/test_utils.jl	3	85.31%
src/model_utils.jl	3	19.64%
src/varnamedvector.jl	3	89.42%
src/distribution_wrappers.jl	4	44.12%
ext/DynamicPPLForwardDiffExt.jl	4	66.67%

Totals
Change from base Build 11501458234:	0.01%
Covered Lines:	3339
Relevant Lines:	4193

---

💛 - Coveralls

[mhauru]

Thanks for looking into this!

[torfjelde]

Great stuff @penelopeysm :)