[Errors] Error on unavailable combinations (#142)

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* bugs

* bugs

* bugs

* bugs

* bugs

* bugs

* bugs

benchmarl/experiment/experiment.py

@@ -26,10 +26,13 @@
 from torchrl.record.loggers import generate_exp_name
 from tqdm import tqdm
 
+from benchmarl.algorithms import IppoConfig, MappoConfig
+
 from benchmarl.algorithms.common import AlgorithmConfig
 from benchmarl.environments import Task
 from benchmarl.experiment.callback import Callback, CallbackNotifier
 from benchmarl.experiment.logger import Logger
+from benchmarl.models import GnnConfig, SequenceModelConfig
 from benchmarl.models.common import ModelConfig
 from benchmarl.utils import _read_yaml_config, seed_everything
 
@@ -361,7 +364,30 @@ def _setup(self):
         self._on_setup()
 
     def _perfrom_checks(self):
-        pass
+        for config in (self.model_config, self.critic_model_config):
+            if isinstance(config, SequenceModelConfig):
+                for layer_config in config.model_configs[1:]:
+                    if isinstance(layer_config, GnnConfig) and (
+                        layer_config.position_key is not None
+                        or layer_config.velocity_key is not None
+                    ):
+                        raise ValueError(
+                            "GNNs reading position or velocity keys are currently only usable in first"
+                            " layer of sequence models"
+                        )
+
+        if self.algorithm_config in (MappoConfig, IppoConfig):
+            critic_model_config = self.critic_model_config
+            if isinstance(critic_model_config, SequenceModelConfig):
+                critic_model_config = self.critic_model_config.model_configs[0]
+            if (
+                isinstance(critic_model_config, GnnConfig)
+                and critic_model_config.topology == "from_pos"
+            ):
+                raise ValueError(
+                    "GNNs in PPO critics with topology 'from_pos' are currently not available, "
+                    "see https://github.com/pytorch/rl/issues/2537"
+                )
 
     def _set_action_type(self):
         if (