Fix UndefVarError: ST not defined

[devmotion]

A large number of errors in TuringLang/DistributionsAD.jl#254 are caused by an UndefVarError: ST not defined. The logs (see, e.g., https://github.com/TuringLang/DistributionsAD.jl/actions/runs/5872312798/job/16026479059#step:6:47460 https://github.com/TuringLang/DistributionsAD.jl/actions/runs/5872312798/job/16026479059#step:6:47460) point to line 2619 in src/compiler.jl.

This PR ensures that variable ST is defined by copying definitions from other functions in src/compiler.jl - since I don't know what they are doing, I don't know if this suggestion is correct.

While looking at src/compiler.jl, I noticed that the same problem should be present in line 2640, and hence I applied the same suggestions there as well.

I don't have a simple MWE, so no test is added.

[codecov-commenter]

Codecov Report

Patch coverage has no change and project coverage change: -0.04% ⚠️

Comparison is base (026696b) 77.49% compared to head (f4dc70c) 77.46%.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1019      +/-   ##
==========================================
- Coverage   77.49%   77.46%   -0.04%     
==========================================
  Files          19       19              
  Lines        8892     8896       +4     
==========================================
  Hits         6891     6891              
- Misses       2001     2005       +4     

Files Changed	Coverage Δ
src/compiler.jl	78.28% <0.00%> (-0.06%)	⬇️

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[wsmoses]

FYI @devmotion this is actually an error in the error handler code.

Specifically the code meant to apply to a type unstable splat in vector mode (which is presently unsupported). This will get past this, but also likely indicates there is a part of the code that cannot currently be differentiated (which hopefully now will be identified by a backtrace).