Fixes CPU and hanging GPU crash (#2118)

* training batch clean up

* training batch clean up

* training batch clean up

Author: williamFalcon

pytorch_lightning/trainer/trainer.py

@@ -1033,7 +1033,7 @@ def run_pretrain_routine(self, model: LightningModule):
                 self.early_stop_callback._validate_condition_metric(callback_metrics)
 
         # clear cache before training
-        if self.on_gpu:
+        if self.on_gpu and self.root_gpu is not None:
             # use context because of:
             # https://discuss.pytorch.org/t/out-of-memory-when-i-use-torch-cuda-empty-cache/57898
             with torch.cuda.device(f'cuda:{self.root_gpu}'):