Add --logdir argument to sample.py parser and restore it in app.py

sample.py

@@ -342,6 +342,7 @@ if __name__ == '__main__':
     # Model config
     parser.add_argument('--model_config_path', required=True, type=str, help='path to YAML config file or directory containing config file')
     parser.add_argument('--checkpoint_path', required=True, type=str, help='path to local checkpoint file (required when using model_config_path)')
+    parser.add_argument('--logdir', default='./logs', type=pathlib.Path, help='output directory for generated samples')
     parser.add_argument('--save_folder', default=0, type=int)
 
     # Caption, condition