[Model offload] Add nice warning (#2543)
* [Model offload] Add nice warning
* Treat sequential and model offload differently.
Sequential raises an error because the operation would fail with a
cryptic warning later.
* Forcibly move to cpu when offloading.
* make style
* one more fix
* make fix-copies
* up
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Co-authored-by: Pedro Cuenca <pedro@huggingface.co>
