This answer said DataLoader will use default_collate to handle Dataset’s return value implicitly when collate_fn is undefined, maybe. But DataLoader’s default argument for collate_fn is actually None, inducing developer hard to find out that DataLoader will use default_collate implicitly when collate_fn is undefined.
The collate_fn is either set to detault_collate or default_convert based on the internal attribute self._auto_collation as seen here which is most likely the reason it’s set to None.