v2: Algorithm - don't initialize nn.Module if it already was initialized

Necessary to permit clean diamond inheritance, as nn.Module.__init__ uses setattr and overwrites members

Author: MischaPanch

tianshou/algorithm/algorithm_base.py

@@ -450,7 +450,13 @@ def __init__(
         policy: TPolicy,
     ) -> None:
         """:param policy: the policy"""
-        super().__init__()
+        module_already_initialized = hasattr(self, "_modules")
+        if not module_already_initialized:
+            super().__init__()
+        else:
+            logger.debug(
+                f"Not initializing nn.Module in {self.__class__.__name__} since it is already initialized (probably due to diamond inheritance)."
+            )
         self.policy: TPolicy = policy
         self.lr_schedulers: list[LRScheduler] = []
         self._optimizers: list["Algorithm.Optimizer"] = []