refactor: replace WeakValueDict with WeakKeyDict

Author: bowenszhu

src/types.jl

@@ -528,15 +528,15 @@ Implements hash consing (flyweight design pattern) for `BasicSymbolic` objects.
 
 This function checks if an equivalent `BasicSymbolic` object already exists. It uses a 
 custom hash function (`hash2`) incorporating metadata and symtypes to search for existing 
-objects in a `WeakValueDict` (`wvd`).  Due to the possibility of hash collisions (where 
+objects in a `WeakKeyDict` (`wkd`). Due to the possibility of hash collisions (where 
 different objects produce the same hash), a custom equality check (`isequal_with_metadata`) 
 which includes metadata comparison, is used to confirm the equivalence of objects with 
 matching hashes. If an equivalent object is found, the existing object is returned; 
 otherwise, the input `s` is returned. This reduces memory usage, improves compilation time 
 for runtime code generation, and supports built-in common subexpression elimination, 
 particularly when working with symbolic objects with metadata.
 
-Using a `WeakValueDict` ensures that only weak references to `BasicSymbolic` objects are 
+Using a `WeakKeyDict` ensures that only weak references to `BasicSymbolic` objects are 
 stored, allowing objects that are no longer strongly referenced to be garbage collected. 
 Custom functions `hash2` and `isequal_with_metadata` are used instead of `Base.hash` and 
 `Base.isequal` to accommodate metadata without disrupting existing tests reliant on the 
@@ -546,12 +546,13 @@ function BasicSymbolic(s::BasicSymbolic)::BasicSymbolic
     if !ENABLE_HASHCONSING[]
         return s
     end
-    h = hash2(s)
-    t = get!(wvd[], h, s)
-    if t === s || isequal_with_metadata(t, s)
-        t
+    hcw = HashConsingWrapper(s)
+    k = getkey(wkd[], hcw, nothing)
+    if isnothing(k)
+        wkd[][hcw] = nothing
+        return s
     else
-        s
+        return k.bs
     end
 end