[rllib] Remove legacy multiagent support (#2975)

* remove legacy

* remove reshaper

Author: ericl

python/ray/rllib/examples/legacy_multiagent/__init__.py

@@ -17,7 +17,6 @@
 from ray.rllib.models.fcnet import FullyConnectedNetwork
 from ray.rllib.models.visionnet import VisionNetwork
 from ray.rllib.models.lstm import LSTM
-from ray.rllib.models.multiagentfcnet import MultiAgentFullyConnectedNetwork
 
 MODEL_CONFIGS = [
     # === Built-in options ===
@@ -178,13 +177,6 @@ def _get_model(inputs, num_outputs, options, state_in, seq_lens):
 
         obs_rank = len(inputs.shape) - 1
 
-        # num_outputs > 1 used to avoid hitting this with the value function
-        if isinstance(
-                options.get("custom_options", {}).get(
-                    "multiagent_fcnet_hiddens", 1), list) and num_outputs > 1:
-            return MultiAgentFullyConnectedNetwork(inputs, num_outputs,
-                                                   options)
-
         if obs_rank > 1:
             return VisionNetwork(inputs, num_outputs, options)