Revert "Fix Tracker gradient through solve on RecursiveArrayTools v4"#3664
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…3665) * Fix Tracker gradient on RAT v4 by preserving the ODESolution wrapper In RecursiveArrayTools v4 `AbstractVectorOfArray <: AbstractArray`, so the `sol isa AbstractArray` branch in `Tracker.@Grad function DiffEqBase.solve_up` now matches ODESolution and returns the nested `sol.u :: Vector{Vector{Float64}}` directly. Tracker tracks the vector-of-vectors, and downstream `sum(solve(...))` reduces the outer vector element-wise into a `Vector{Float64}`, breaking `Tracker.gradient(loss, p)` callers with "Function output is not scalar". Return the ODESolution wrapper itself for `AbstractVectorOfArray` inputs so callers reduce through the RAT v4 AbstractArray interface and get a scalar as before. Earlier attempt (#3663) stacked into a fresh matrix via `Array(sol)`; that was reverted in #3664 because it changed the return type and broke downstream consumers. Preserving the wrapper keeps the contract. Refs SciML/SciMLSensitivity.jl#1331. Co-Authored-By: Chris Rackauckas <accounts@chrisrackauckas.com> * Update DiffEqBaseTrackerExt.jl --------- Co-authored-by: ChrisRackauckas-Claude <accounts@chrisrackauckas.com>
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Reverts #3663