Better default for offload_state_dict in from_pretrained
# What does this PR do?

Seeing issues arise since the release of big model inference, I realized it's very confusing for users to have to set `offload_state_dict=True` when the device map picked with `device_map="auto"` contains some disk-offloaded weights. Therefore, this PR changes the default to `None` to pick a good default (basically `False` if there is no disk offload and `True` otherwise) while still letting the user choose the behavior they want by passing a value.
_The documentation is not available anymore as the PR was closed or merged._