Expose mj_makeM as a sub-component.

PiperOrigin-RevId: 763662068
Change-Id: I55ced456dca751c1de09fb35e2f73c60ffc0d626

include/mujoco/mjdata.h

@@ -295,7 +295,7 @@ struct mjData_ {
   int*    moment_colind;     // column indices in sparse Jacobian                (nJmom x 1)
   mjtNum* actuator_moment;   // actuator moments                                 (nJmom x 1)
 
-  // computed by mj_fwdPosition/mj_crb
+  // computed by mj_fwdPosition/mj_makeM
   mjtNum* crb;               // com-based composite inertia and mass             (nbody x 10)
   mjtNum* qM;                // inertia (sparse)                                 (nM x 1)
   mjtNum* M;                 // reduced inertia (compressed sparse row)          (nC x 1)

include/mujoco/mujoco.h

@@ -364,6 +364,9 @@ MJAPI void mj_transmission(const mjModel* m, mjData* d);
 // Run composite rigid body inertia algorithm (CRB).
 MJAPI void mj_crb(const mjModel* m, mjData* d);
 
+// Make inertia matrix.
+MJAPI void mj_makeM(const mjModel* m, mjData* d);
+
 // Compute sparse L'*D*L factorizaton of inertia matrix.
 MJAPI void mj_factorM(const mjModel* m, mjData* d);