fix: correct gradients for vector of symbols

ext/SciMLBaseZygoteExt.jl

@@ -120,6 +120,29 @@ end
     VA[sym], ODESolution_getindex_pullback
 end
 
+@adjoint function getindex(VA::ODESolution{T}, sym::Union{Tuple, AbstractVector}) where T
+    function ODESolution_getindex_pullback(Δ)
+        sym = sym isa Tuple ? collect(sym) : sym
+        i = map(x -> symbolic_type(x) != NotSymbolic() ? variable_index(VA, x) : x, sym)
+        if i === nothing
+            throw(error("Zygote AD of purely-symbolic slicing for observed quantities is not yet supported. Work around this by using `A[sym,i]` to access each element sequentially in the function being differentiated."))
+        else
+            Δ′ = map(enumerate(VA.u)) do (t_idx, us)
+                map(enumerate(us)) do (u_idx, u)
+                    if u_idx in i
+                        idx = findfirst(isequal(u_idx), i)
+                        Δ[t_idx][idx]
+                    else
+                        zero(T)
+                    end
+                end
+            end
+            (Δ′, nothing)
+        end
+    end
+    VA[sym], ODESolution_getindex_pullback
+end
+
 @adjoint function ODESolution{T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12
 }(u,
         args...) where {T1, T2, T3, T4, T5, T6, T7, T8,