fix: add adjoints for symbolic indexing with ::Colon

ext/SciMLBaseZygoteExt.jl

@@ -125,6 +125,32 @@ end
     VA[sym], ODESolution_getindex_pullback
 end
 
+@adjoint function Base.getindex(VA::ODESolution, sym, ::Colon)
+    function ODESolution_getindex_pullback(Δ)
+        i = symbolic_type(sym) != NotSymbolic() ? variable_index(VA, sym) : sym
+        if is_observed(VA, sym)
+            f = observed(VA, sym)
+            p = parameter_values(VA)
+            tunables, _, _ = SciMLStructures.canonicalize(SciMLStructures.Tunable(), p)
+            u = state_values(VA)
+            t = current_time(VA)
+            y, back = Zygote.pullback(u, tunables) do u, tunables
+                f.(u, Ref(tunables), t)
+            end
+            gs = back(Δ)
+            (gs[1], nothing)
+        elseif 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
+            @show i
+            Δ′ = [[i == k ? Δ[j] : zero(x[1]) for k in 1:length(x)]
+                  for (x, j) in zip(VA.u, 1:length(VA))]
+            (Δ′, nothing)
+        end
+    end
+    VA[sym, :], ODESolution_getindex_pullback
+end
+
 function obs_grads(VA, sym, obs_idx, Δ)
     y, back = Zygote.pullback(VA) do sol
         getindex.(Ref(sol), sym[obs_idx])
@@ -172,6 +198,25 @@ end
     VA[sym], ODESolution_getindex_pullback
 end
 
+@adjoint function Base.getindex(
+        VA::ODESolution{T}, sym::Union{Tuple, AbstractVector}, ::Colon) 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)
+
+        obs_idx = findall(s -> is_observed(VA, s), sym)
+        not_obs_idx = setdiff(1:length(sym), obs_idx)
+
+        gs_obs = obs_grads(VA, sym, isempty(obs_idx) ? nothing : obs_idx, Δ)
+        gs_not_obs = not_obs_grads(VA, sym, not_obs_idx, i, Δ)
+
+        a = Zygote.accum(gs_obs[1], gs_not_obs)
+
+        (a, nothing)
+    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,