use gfile to support remote directories

pytorch_lightning/callbacks/model_checkpoint.py

@@ -101,7 +101,7 @@ def __init__(self, filepath: Optional[str] = None, monitor: str = 'val_loss', ve
                  save_last: bool = False, save_top_k: int = 1, save_weights_only: bool = False,
                  mode: str = 'auto', period: int = 1, prefix: str = ''):
         super().__init__()
-        if save_top_k > 0 and filepath is not None and tf.io.gfile.isdir(filepath) and len(os.listdir(filepath)) > 0:
+        if save_top_k > 0 and filepath is not None and tf.io.gfile.isdir(filepath) and len(tf.io.gfile.listdir(filepath)) > 0:
             rank_zero_warn(
                 f"Checkpoint directory {filepath} exists and is not empty with save_top_k != 0."
                 "All files in this directory will be deleted when a checkpoint is saved!"