Resume from last checkpoint by default if exists in output_folder

src/ptbench/utils/checkpointer.py

 import logging
 import os
 
+import typing
 logger = logging.getLogger(__name__)
 
 
-def get_checkpoint(output_folder, resume_from):
+def get_checkpoint(output_folder: str, resume_from: typing.Literal["last", "best"] | str | None) -> str | None : 
     """Gets a checkpoint file.
 
     Can return the best or last checkpoint, or a checkpoint at a specific path.
     Ensures the checkpoint exists, raising an error if it is not the case.
 
+    If resume_from is None, checks the output directory if a checkpoint already exists and returns it.
+    If no checkpoint is found, returns None.
+
     Parameters
     ----------
 
-    output_folder : :py:class:`str`
+    output_folder:
         Directory in which checkpoints are stored.
 
-    resume_from : :py:class:`str`
+    resume_from:
         Which model to get. Can be one of "best", "last", or a path to a checkpoint.
+        If None, gets the last checkpoint if it exists, otherwise returns None
 
     Returns
     -------
 
-    checkpoint_file : :py:class:`str`
-        The requested model.
+    checkpoint_file:
+        Path to the requested checkpoint or None.
     """
     last_checkpoint_path = os.path.join(output_folder, "model_final_epoch.ckpt")
     best_checkpoint_path = os.path.join(
@@ -49,7 +54,11 @@ def get_checkpoint(output_folder, resume_from):
             )
 
     elif resume_from is None:
-        checkpoint_file = None
+        if os.path.isfile(last_checkpoint_path):
+            checkpoint_file = last_checkpoint_path
+            logger.info(f"Found existing checkpoint {last_checkpoint_path}. Loading.")
+        else:
+            return None
 
     else:
         if os.path.isfile(resume_from):