Update default filename (#4500)

Co-authored-by: Jirka Borovec <Borda@users.noreply.github.com>
Co-authored-by: Jeff Yang <ydcjeff@outlook.com>

pytorch_lightning/callbacks/model_checkpoint.py

@@ -366,7 +366,8 @@ class ModelCheckpoint(Callback):
     ) -> str:
         if not filename:
             # filename is not set, use default name
-            filename = "{epoch}-{step}"
+            filename = "{epoch}" + cls.CHECKPOINT_JOIN_CHAR + "{step}"
+
         # check and parse user passed keys in the string
         groups = re.findall(r"(\{.*?)[:\}]", filename)
         if len(groups) >= 0: