Forbid PretrainedConfig from saving generate parameters; Update deprecations in generate-related code 🧹 (#32659)

Co-authored-by: amyeroberts <22614925+amyeroberts@users.noreply.github.com>

tests/models/decision_transformer/test_modeling_decision_transformer.py

@@ -41,7 +41,6 @@ class DecisionTransformerModelTester:
         act_dim=6,
         state_dim=17,
         hidden_size=23,
-        max_length=11,
         is_training=True,
     ):
         self.parent = parent
@@ -50,7 +49,6 @@ class DecisionTransformerModelTester:
         self.act_dim = act_dim
         self.state_dim = state_dim
         self.hidden_size = hidden_size
-        self.max_length = max_length
         self.is_training = is_training
 
     def prepare_config_and_inputs(self):
@@ -80,7 +78,6 @@ class DecisionTransformerModelTester:
             act_dim=self.act_dim,
             state_dim=self.state_dim,
             hidden_size=self.hidden_size,
-            max_length=self.max_length,
         )
 
     def create_and_check_model(