Fix Trainer when model is loaded on a different GPU
# What does this PR do?

When a small model is loaded with `device_map="auto"` it might end up all on GPU 1, so currently `is_model_parallel` is set to `False` (cause one device) and later on the Trainer moves the model to GPU 0 which fails the execution of all the Accelerate hooks.

This PR fixes this by making sure `is_model_parallel` is set to `True` when there is one device but it's not GPU 0.
_The documentation is not available anymore as the PR was closed or merged._