[Migration] forawrd dynamics (Genesis-Embodied-AI#1406)

Author: YilingQiao

genesis/engine/solvers/rigid/array_class.py

@@ -43,6 +43,17 @@ def __init__(self, solver, n_dofs: int, n_entities: int, n_geoms: int, _B: int,
 # =========================================== Collider ===========================================
 
 
+@ti.data_oriented
+class ConstraintState:
+    """
+    Class to store the mutable constraint data, all of which type is [ti.fields].
+    """
+
+    def __init__(self, solver):
+        f_batch = solver._batch_shape
+        self.n_constraints = ti.field(dtype=gs.ti_int, shape=f_batch())
+
+
 @ti.data_oriented
 class ColliderState:
     """