feat(minitaur): add minitaur step/reset reference info return

This commit adds the `reference`, `state_of_interest` and `reference_error` to the info dictionary
that is returned by the environment's step and reset methods.

stable_gym/envs/robotics/minitaur/minitaur_bullet_cost/minitaur_bullet_cost.py

@@ -349,7 +349,7 @@ def step(self, action):
         self.state = obs
         self.t = self.t + self.dt
 
-        # Retrieve original rew5ards and base velocity.
+        # Retrieve original rewards.
         # NOTE: Han et al. 2018 used the squared error for the drift reward. We use the
         # version found in the original Minitaur environment (i.e. absolute distance).
         objectives = super().get_objectives()
@@ -359,9 +359,16 @@ def step(self, action):
 
         # Compute the cost and update the info dict.
         cost, cost_info = self.cost(
-            self.base_velocity, energy_reward, drift_cost, shake_cost
+            base_velocity, energy_reward, drift_cost, shake_cost
         )
         info.update(cost_info)
+        info.update(
+            {
+                "reference": self.reference_forward_velocity,
+                "state_of_interest": base_velocity,
+                "reference_error": base_velocity - self.reference_forward_velocity,
+            }
+        )
 
         # Add optional health penalty at the end of the episode if requested.
         if self._include_health_penalty:

tests/__snapshots__/test_minitaur_cost.ambr

@@ -17,6 +17,9 @@
     'cost_shake': 0.001545201927514,
     'cost_velocity': 1.262878009055252,
     'energy_cost': 0.208187038300126,
+    'reference': 1.0,
+    'reference_error': -1.123778451944712,
+    'state_of_interest': -0.123778451944712,
   })
 # ---
 # name: TestMinitaurBulletCostEqual.test_snapshot.102
@@ -136,6 +139,9 @@
     'cost_shake': 0.001216954533656,
     'cost_velocity': 1.308016924484482,
     'energy_cost': 0.411224652260461,
+    'reference': 1.0,
+    'reference_error': -1.143685675561464,
+    'state_of_interest': -0.143685675561464,
   })
 # ---
 # name: TestMinitaurBulletCostEqual.test_snapshot.137
@@ -258,6 +264,9 @@
     'cost_shake': 0.001062425280662,
     'cost_velocity': 1.05929279162964,
     'energy_cost': 0.411447438208018,
+    'reference': 1.0,
+    'reference_error': -1.029219506047976,
+    'state_of_interest': -0.029219506047976,
   })
 # ---
 # name: TestMinitaurBulletCostEqual.test_snapshot.172
@@ -380,6 +389,9 @@
     'cost_shake': 0.002593163820677,
     'cost_velocity': 1.011816047403776,
     'energy_cost': 0.457486833811053,
+    'reference': 1.0,
+    'reference_error': -1.005890673683664,
+    'state_of_interest': -0.005890673683664,
   })
 # ---
 # name: TestMinitaurBulletCostEqual.test_snapshot.21
@@ -536,6 +548,9 @@
     'cost_shake': 0.158954162148827,
     'cost_velocity': 1.147753757228502,
     'energy_cost': 0.201663962061444,
+    'reference': 1.0,
+    'reference_error': -1.071332701465097,
+    'state_of_interest': -0.071332701465097,
   })
 # ---
 # name: TestMinitaurBulletCostEqual.test_snapshot.67