Move OpenUSD parsing to usd_decoder

Add usd_decoder to mujoco/plugin.
  - Previously was src/experimental/usd/usd_to_mjspec.cc, now that same spec is returned by the plugin.

PiperOrigin-RevId: 854350962
Change-Id: Ia980190a557c50ce8197980922a2594b613859b4

src/xml/xml_api.cc

@@ -31,12 +31,6 @@
 #include "xml/xml_global.h"
 #include "xml/xml_native_reader.h"
 #include "xml/xml_util.h"
-#if defined(mjUSEUSD)
-#include <mujoco/experimental/usd/usd.h>
-#include <pxr/usd/usd/stage.h>
-#endif
-
-
 
 //---------------------------------- Functions -----------------------------------------------------
 
@@ -75,36 +69,6 @@ mjModel* mj_loadXML(const char* filename, const mjVFS* vfs,
   return m;
 }
 
-#if defined(mjUSEUSD)
-//  parse USD file, compile it, and return low-level model.
-//  if vfs is not NULL, look up files in vfs before reading from disk
-//  error can be NULL; otherwise assumed to have size error_sz
-mjModel* mj_loadUSD(const char* filename, const mjVFS* vfs, char* error, int error_sz) {
-  auto stage = pxr::UsdStage::Open(filename);
-
-  if (stage == nullptr) {
-    mjCopyError(error, "Failed to load USD stage from file.", error_sz);
-    return nullptr;
-  }
-
-  // Parse USD into mjSpec.
-  std::unique_ptr<mjSpec, std::function<void(mjSpec*)> > spec(
-    mj_parseUSDStage(stage),
-    [](mjSpec* s) {
-      mj_deleteSpec(s);
-    });
-
-  // Compile new model.
-  mjModel* m = mj_compile(spec.get(), vfs);
-  if (!m) {
-    mjCopyError(error, mjs_getError(spec.get()), error_sz);
-    return nullptr;
-  }
-
-  SetGlobalXmlSpec(spec.release());
-  return m;
-}
-#endif
 
 // update XML data structures with info from low-level model, save as MJCF
 //  returns 1 if successful, 0 otherwise