[tune] Disallow setting resources_per_trial when it is already configured (#4880)

* disallow it

* import fix

* fix example

* fix test

* fix tests

* Update mock.py

* fix

* make less convoluted

* fix tests

python/ray/rllib/agents/mock.py

@@ -20,6 +20,10 @@ class _MockTrainer(Trainer):
         "num_workers": 0,
     })
 
+    @classmethod
+    def default_resource_request(cls, config):
+        return None
+
     def _init(self, config, env_creator):
         self.info = None
         self.restored = False

python/ray/rllib/examples/custom_train_fn.py

@@ -40,13 +40,9 @@ def my_train_fn(config, reporter):
 
 if __name__ == "__main__":
     ray.init()
-    tune.run(
-        my_train_fn,
-        resources_per_trial={
-            "cpu": 1,
-        },
-        config={
-            "lr": 0.01,
-            "num_workers": 0,
-        },
-    )
+    config = {
+        "lr": 0.01,
+        "num_workers": 0,
+    }
+    resources = PPOTrainer.default_resource_request(config).to_json()
+    tune.run(my_train_fn, resources_per_trial=resources, config=config)