fix: fix intialization of array parameters with unknown size

Author: AayushSabharwal

src/systems/parameter_buffer.jl

@@ -18,6 +18,8 @@ function MTKParameters(
         error("Cannot create MTKParameters if system does not have index_cache")
     end
     all_ps = Set(unwrap.(full_parameters(sys)))
+    arr_ps = Set(arguments(p)[1] for p in all_ps if istree(p) && operation(p) === getindex)
+    all_ps = union(all_ps, arr_ps)
     union!(all_ps, default_toterm.(unwrap.(full_parameters(sys))))
     if p isa Vector && !(eltype(p) <: Pair) && !isempty(p)
         ps = full_parameters(sys)
@@ -93,7 +95,14 @@ function MTKParameters(
         val = symconvert(ctype, unwrap(fixpoint_sub(val, p)))
         done = set_value(sym, val)
         if !done && Symbolics.isarraysymbolic(sym)
-            done = all(set_value.(collect(sym), val))
+            if Symbolics.shape(sym) === Symbolics.Unknown()
+                done = all(set_value(sym[i], val[i]) for i in eachindex(val))
+            else
+                if size(sym) != size(val)
+                    error("Got value of size $(size(val)) for parameter $sym of size $(size(sym))")
+                end
+                done = all(set_value.(collect(sym), val))
+            end
         end
         if !done
             error("Symbol $sym does not have an index")

test/mtkparameters.jl

@@ -101,3 +101,14 @@ function loss(value, sys, ps)
 end
 
 @test ForwardDiff.derivative(x -> loss(x, sys, ps), 1.5) == 3.0
+
+# Issue#2615
+@parameters p::Vector{Float64}
+@variables X(t)
+eq = D(X) ~ p[1] - p[2] * X
+@mtkbuild osys = ODESystem([eq], t)
+
+u0 = [X => 1.0]
+ps = [p => [2.0, 0.1]]
+p = MTKParameters(osys, ps, u0)
+@test p.tunable[1] == [2.0, 0.1]