reset ppo learn.py to master

Project/Assets/ML-Agents/Examples/Walker/Demos/ExpertWalkerDy.demo.meta

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   userData: ' (Unity.MLAgents.Demonstrations.DemonstrationSummary)'
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Project/Assets/ML-Agents/Examples/Walker/Demos/ExpertWalkerDyVS.demo.meta

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Project/Assets/ML-Agents/Examples/Walker/Demos/ExpertWalkerSt.demo.meta

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 ScriptedImporter:
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Project/Assets/ML-Agents/Examples/Walker/Demos/ExpertWalkerStVS.demo.meta

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ml-agents/mlagents/trainers/learn.py

 from mlagents.trainers.trainer_util import TrainerFactory, handle_existing_directories
 from mlagents.trainers.stats import (
     TensorboardWriter,
-    CSVWriter,
     StatsReporter,
     GaugeWriter,
     ConsoleWriter,
                 os.path.join(run_logs_dir, "training_status.json")
             )
 
-        # Configure CSV, Tensorboard Writers and StatsReporter
-        # We assume reward and episode length are needed in the CSV.
-        csv_writer = CSVWriter(
-            write_path,
-            required_fields=[
-                "Environment/Cumulative Reward",
-                "Environment/Episode Length",
-            ],
-        )
+        # Configure Tensorboard Writers and StatsReporter
         tb_writer = TensorboardWriter(
             write_path, clear_past_data=not checkpoint_settings.resume
         )
-        StatsReporter.add_writer(csv_writer)
         StatsReporter.add_writer(gauge_write)
         StatsReporter.add_writer(console_writer)
 
 
 
 def create_environment_factory(
-    env_path: Optional[str],
-    no_graphics: bool,
-    seed: int,
-    start_port: Optional[int],
-    env_args: Optional[List[str]],
-    log_folder: str,
+        env_path: Optional[str],
+        no_graphics: bool,
+        seed: int,
+        start_port: Optional[int],
+        env_args: Optional[List[str]],
+        log_folder: str,
-        worker_id: int, side_channels: List[SideChannel]
+            worker_id: int, side_channels: List[SideChannel]
     ) -> UnityEnvironment:
         # Make sure that each environment gets a different seed
         env_seed = seed + worker_id