Move get_x_checkpoint functions to utils/checkpoint.py

Reviewed By: JKSenthil

Differential Revision: D56450720

Author: diego-urgell

tests/framework/callbacks/test_base_checkpointer.py

@@ -249,7 +249,7 @@ def test_restore_from_latest_empty_dir(self) -> None:
                 self.assertEqual(
                     log.output,
                     [
-                        f"WARNING:torchtnt.framework.callbacks._checkpoint_utils:Input dirpath doesn't contain any subdirectories: {temp_dir}"
+                        f"WARNING:torchtnt.utils.checkpoint:Input dirpath doesn't contain any subdirectories: {temp_dir}"
                     ],
                 )
                 self.assertFalse(restored)