[Fix] Fix no attribute 'samples_per_gpu' bug in `auto_scale_lr` (#7862)

* Fix

* update docstring

mmdet/apis/train.py

@@ -96,10 +96,10 @@ def auto_scale_lr(cfg, distributed, logger):
         num_gpus = len(cfg.gpu_ids)
 
     # calculate the batch size
-    batch_size = num_gpus * cfg.data.samples_per_gpu
-    logger.info(f'You are using {num_gpus} GPU(s) '
-                f'and {cfg.data.samples_per_gpu} samples per GPU. '
-                f'Total batch size is {batch_size}.')
+    samples_per_gpu = cfg.data.train_dataloader.samples_per_gpu
+    batch_size = num_gpus * samples_per_gpu
+    logger.info(f'Training with {num_gpus} GPU(s) with {samples_per_gpu} '
+                f'samples per GPU. The total batch size is {batch_size}.')
 
     if batch_size != base_batch_size:
         # scale LR with

tools/train.py

@@ -125,7 +125,7 @@ def main():
                           '"auto_scale_lr.enable" or '
                           '"auto_scale_lr.base_batch_size" in your'
                           ' configuration file. Please update all the '
-                          'configuration files to mmdet >= 2.23.1.')
+                          'configuration files to mmdet >= 2.24.0.')
 
     # set multi-process settings
     setup_multi_processes(cfg)