Implement returned(model, xs...)

[penelopeysm]

Pushed to main accidentally, then reverted on main. This PR is where it should've been...

Closes #1123 -- but this PR is even more general in that it accepts any combination of arguments (not just a single argument), because there might be a present or future overload out there for InitFromParams(a, b, c, ..., nothing) -- (and indeed there is already one!).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.01%. Comparing base (e3fbd2d) to head (71ef2b2).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1171   +/-   ##
=======================================
  Coverage   80.01%   80.01%           
=======================================
  Files          41       41           
  Lines        3877     3877           
=======================================
  Hits         3102     3102           
  Misses        775      775           

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[github-actions]

Benchmark Report

• this PR's head: 71ef2b2672d536ef31c9daf1b4e140fb0e8fa5f2
• base branch: e3fbd2d856d1f2cbc39e7ddc11ee21f68737776f

Computer Information

Julia Version 1.11.7
Commit f2b3dbda30a (2025-09-08 12:10 UTC)
Build Info:
  Official https://julialang.org/ release
Platform Info:
  OS: Linux (x86_64-linux-gnu)
  CPU: 4 × Intel(R) Xeon(R) Platinum 8370C CPU @ 2.80GHz
  WORD_SIZE: 64
  LLVM: libLLVM-16.0.6 (ORCJIT, icelake-server)
Threads: 1 default, 0 interactive, 1 GC (on 4 virtual cores)

Benchmark Results

┌───────────────────────┬───────┬─────────────┬───────────────────┬────────┬────────────────────────────────┬───────────────────────────┬─────────────────────────────────┐
│                       │       │             │                   │        │        t(eval) / t(ref)        │     t(grad) / t(eval)     │        t(grad) / t(ref)         │
│                       │       │             │                   │        │ ──────────┬──────────┬──────── │ ──────┬─────────┬──────── │ ──────────┬───────────┬──────── │
│                 Model │   Dim │  AD Backend │           VarInfo │ Linked │      base │  this PR │ speedup │  base │ this PR │ speedup │      base │   this PR │ speedup │
├───────────────────────┼───────┼─────────────┼───────────────────┼────────┼───────────┼──────────┼─────────┼───────┼─────────┼─────────┼───────────┼───────────┼─────────┤
│               Dynamic │    10 │    mooncake │             typed │   true │    362.23 │   336.75 │    1.08 │ 10.08 │   11.68 │    0.86 │   3649.94 │   3933.20 │    0.93 │
│                   LDA │    12 │ reversediff │             typed │   true │   2663.53 │  2411.99 │    1.10 │  4.89 │    5.01 │    0.98 │  13032.35 │  12073.13 │    1.08 │
│   Loop univariate 10k │ 10000 │    mooncake │             typed │   true │ 103823.85 │ 94757.11 │    1.10 │  3.97 │    4.02 │    0.99 │ 411999.66 │ 380934.63 │    1.08 │
├───────────────────────┼───────┼─────────────┼───────────────────┼────────┼───────────┼──────────┼─────────┼───────┼─────────┼─────────┼───────────┼───────────┼─────────┤
│    Loop univariate 1k │  1000 │    mooncake │             typed │   true │   8511.29 │  7307.17 │    1.16 │  4.65 │    4.80 │    0.97 │  39585.13 │  35045.24 │    1.13 │
│      Multivariate 10k │ 10000 │    mooncake │             typed │   true │  31689.53 │ 28801.57 │    1.10 │ 10.41 │   10.88 │    0.96 │ 329961.83 │ 313273.76 │    1.05 │
│       Multivariate 1k │  1000 │    mooncake │             typed │   true │   3488.41 │  3291.08 │    1.06 │  9.64 │   12.99 │    0.74 │  33620.80 │  42749.28 │    0.79 │
├───────────────────────┼───────┼─────────────┼───────────────────┼────────┼───────────┼──────────┼─────────┼───────┼─────────┼─────────┼───────────┼───────────┼─────────┤
│ Simple assume observe │     1 │ forwarddiff │             typed │  false │      2.75 │     2.47 │    1.11 │  3.85 │    3.99 │    0.96 │     10.57 │      9.85 │    1.07 │
│           Smorgasbord │   201 │ forwarddiff │             typed │  false │   1229.34 │  1125.09 │    1.09 │ 66.33 │  121.00 │    0.55 │  81540.75 │ 136135.65 │    0.60 │
│           Smorgasbord │   201 │ forwarddiff │       simple_dict │   true │       err │      err │     err │   err │     err │     err │       err │       err │     err │
├───────────────────────┼───────┼─────────────┼───────────────────┼────────┼───────────┼──────────┼─────────┼───────┼─────────┼─────────┼───────────┼───────────┼─────────┤
│           Smorgasbord │   201 │ forwarddiff │ simple_namedtuple │   true │       err │      err │     err │   err │     err │     err │       err │       err │     err │
│           Smorgasbord │   201 │      enzyme │             typed │   true │   1672.13 │  1538.04 │    1.09 │  7.22 │    5.14 │    1.40 │  12076.60 │   7913.18 │    1.53 │
│           Smorgasbord │   201 │    mooncake │             typed │   true │   1668.97 │  1531.51 │    1.09 │  5.41 │    5.36 │    1.01 │   9021.10 │   8203.67 │    1.10 │
├───────────────────────┼───────┼─────────────┼───────────────────┼────────┼───────────┼──────────┼─────────┼───────┼─────────┼─────────┼───────────┼───────────┼─────────┤
│           Smorgasbord │   201 │ reversediff │             typed │   true │   1660.36 │  1550.37 │    1.07 │ 95.46 │   88.67 │    1.08 │ 158503.62 │ 137471.51 │    1.15 │
│           Smorgasbord │   201 │ forwarddiff │      typed_vector │   true │   1669.76 │  1540.94 │    1.08 │ 61.47 │   54.61 │    1.13 │ 102643.82 │  84145.73 │    1.22 │
│           Smorgasbord │   201 │ forwarddiff │           untyped │   true │   1669.76 │  1565.59 │    1.07 │ 60.59 │   58.46 │    1.04 │ 101175.34 │  91518.31 │    1.11 │
├───────────────────────┼───────┼─────────────┼───────────────────┼────────┼───────────┼──────────┼─────────┼───────┼─────────┼─────────┼───────────┼───────────┼─────────┤
│           Smorgasbord │   201 │ forwarddiff │    untyped_vector │   true │   1676.95 │  1546.02 │    1.08 │ 61.89 │   56.42 │    1.10 │ 103787.34 │  87223.11 │    1.19 │
│              Submodel │     1 │    mooncake │             typed │   true │      7.25 │     6.59 │    1.10 │  5.45 │    5.19 │    1.05 │     39.48 │     34.22 │    1.15 │
└───────────────────────┴───────┴─────────────┴───────────────────┴────────┴───────────┴──────────┴─────────┴───────┴─────────┴─────────┴───────────┴───────────┴─────────┘