Improve type stability tests, better use of AutoZero backends

[gdalle]

Idea

• In DI, we test correctness and performance of the differentiation mechanism
  • correctness for backends from primitive
  • type stability and weird arrays for zero backends
• In DIT, we test correctness of the tests themselves
  • correctness for ForwardDiff
  • weird arrays for appropriate backends
  • type stability for zero backends

---

DI source

• Add zero backends (AutoZeroForward and AutoZeroReverse) alongside backends from primitive (AutoForwardFromPrimitive and AutoReverseFromPrimitive)
• Replace Returns(y) with a custom ReturnZero(y) to generate a new output every time

DI tests

• Add type stability tests and weird array tests for zero backends
• Remove type stability tests for backends from primitive

DIT source

• Remove zero backends
• Rename scenario_to_zero into Base.zero
• Remove conditionals in type stability tests: pushforward activated even if Bool(pushforward_performance) is false and same for pullback
• Adjust type stability tests and benchmarks so that they work on scenarios with Tangents

DIT tests

• Remove some correctness tests for zero backends

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.84%. Comparing base (7c60378) to head (9a62f3f).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #437      +/-   ##
==========================================
+ Coverage   93.63%   96.84%   +3.21%     
==========================================
  Files         102      103       +1     
  Lines        4885     4884       -1     
==========================================
+ Hits         4574     4730     +156     
+ Misses        311      154     -157     

Flag	Coverage Δ
DI	99.39% <100.00%> (+0.12%)	⬆️
DIT	93.37% <100.00%> (+7.04%)	⬆️

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