Python bindings: Store a reference to MjModel in MjDataWrapper.

Before this change, MjDataWrapper contained a custom data structure called MjDataMetadata, which contained the sizes of various arrays needed to create MjData. This was used for serialization and deserialization, as well as copying MjData instances.

This worked fine for native MuJoCo models, but as soon as plugins were used, there was information in the model that was needed and not available in MjDataMetadata. Since plugins are so general, keeping MjDataMetadata was untenable.

PiperOrigin-RevId: 578205355
Change-Id: I11b9ee797d1da5aaf1fecbbe170b3b0f1f1a3e6c

python/mujoco/functions.cc

@@ -23,6 +23,7 @@
 
 #include <Eigen/Core>
 #include <mujoco/mjxmacro.h>
+#include <mujoco/mujoco.h>
 #include "function_traits.h"
 #include "functions.h"
 #include "private.h"
@@ -1367,7 +1368,7 @@ PYBIND11_MODULE(_functions, pymodule) {
         }
 
 #undef MJ_M
-#define MJ_M(x) d.metadata().x
+#define MJ_M(x) d.model().get()->x
 #undef MJ_D
 #define MJ_D(x) data->x
 #define X(type, name, nr, nc)                                             \
@@ -1379,7 +1380,7 @@ PYBIND11_MODULE(_functions, pymodule) {
   }
 
         MJDATA_ARENA_POINTERS_PRIMAL
-        if (d.metadata().is_dual) {
+        if (mj_isDual(d.model().get())) {
           MJDATA_ARENA_POINTERS_DUAL
         }
 #undef X