[BugFix] make value estimator with value_key from the PPOLoss init arg (#1144)

Author: xmaples

torchrl/objectives/ppo.py

@@ -263,7 +263,7 @@ def make_value_estimator(self, value_type: ValueEstimators = None, **hyperparams
         if hasattr(self, "gamma"):
             hp["gamma"] = self.gamma
         hp.update(hyperparams)
-        value_key = "state_value"
+        value_key = self.value_key
         if value_type == ValueEstimators.TD1:
             self._value_estimator = TD1Estimator(
                 value_network=self.critic, value_key=value_key, **hp