make observables forward diffable

Author: hexaeder

src/symbolicindexing.jl

@@ -555,10 +555,11 @@ function _get_observed_f(im::IndexManager, cf::VertexModel, vidx, _obsf::O) wher
     aggr = im.v_aggr[vidx]
     extr = im.v_out[vidx]
     pr   = im.v_para[vidx]
-    ret = Vector{Float64}(undef, N)
+    retcache = DiffCache(Vector{Float64}(undef, N))
     _hasext = has_external_input(cf)
 
     (u, outbuf, aggbuf, extbuf, p, t) -> begin
+        ret = PreallocationTools.get_tmp(retcache, first(u)*first(p)*first(t))
         ins = if _hasext
             (view(aggbuf, aggr), view(extbuf, extr))
         else

test/symbolicindexing_test.jl

@@ -534,9 +534,9 @@ end
     # @b SII.observed($nw, $(VIndex(2,:Pdamping))) # 15
     # @b SII.observed($nw, $(VIndex(2,:θ))) # 7 5
 
-    b = @b SII.observed($nw, $idxs1) # 69 36 42 30 37
+    b = @b SII.observed($nw, $idxs1) # 69 36 42 30 37 47
     if VERSION ≥ v"1.11"
-        @test b.allocs <= 37
+        @test b.allocs <= 47
     end
     b = @b SII.observed($nw, $idxs2) # 12 7 10 5
     if VERSION ≥ v"1.11"