Update gym_locomotion_envs.py

As suggested in https://github.com/bulletphysics/bullet3/pull/1759. The default isDone lets done = alive<0, and a special case is made for halfcheetah, forcing done=False. 
I had to pass the 'alive' condition as an additive parameter of WalkerBaseBulletEnv.

examples/pybullet/gym/pybullet_envs/gym_locomotion_envs.py

@@ -14,6 +14,7 @@ class WalkerBaseBulletEnv(MJCFBaseBulletEnv):
 		self.walk_target_x = 1e3  # kilometer away
 		self.walk_target_y = 0
 		self.stateId=-1
+		self.alive = None
 		
 
 	def create_single_player_scene(self, bullet_client):
@@ -40,6 +41,12 @@ class WalkerBaseBulletEnv(MJCFBaseBulletEnv):
 		
 		return r
 	
+	def _isDone(self):
+		if self._alive is not None:
+			return self._alive < 0
+		else:
+			return False
+
 	def move_robot(self, init_x, init_y, init_z):
 		"Used by multiplayer stadium to move sideways, to another running lane."
 		self.cpp_robot.query_position()
@@ -60,8 +67,8 @@ class WalkerBaseBulletEnv(MJCFBaseBulletEnv):
 
 		state = self.robot.calc_state()  # also calculates self.joints_at_limit
 
-		alive = float(self.robot.alive_bonus(state[0]+self.robot.initial_z, self.robot.body_rpy[1]))   # state[0] is body height above ground, body_rpy[1] is pitch
+		self.alive = float(self.robot.alive_bonus(state[0]+self.robot.initial_z, self.robot.body_rpy[1]))   # state[0] is body height above ground, body_rpy[1] is pitch
-		done = alive < 0
+		done = self._isDone()
 		if not np.isfinite(state).all():
 			print("~INF~", state)
 			done = True
@@ -136,6 +143,9 @@ class HalfCheetahBulletEnv(WalkerBaseBulletEnv):
 		self.robot = HalfCheetah()
 		WalkerBaseBulletEnv.__init__(self, self.robot)
 		
+	def _isDone(self):
+		return False
+
 class AntBulletEnv(WalkerBaseBulletEnv):
 	def __init__(self):
 		self.robot = Ant()
@@ -172,4 +182,3 @@ class HumanoidFlagrunHarderBulletEnv(HumanoidBulletEnv):
 		s = HumanoidBulletEnv.create_single_player_scene(self, bullet_client)
 		s.zero_at_running_strip_start_line = False
 		return s
-