Limit the use of PreTrainedModel.device
# What does this PR do?

I'm currently working on solutions to do model parallelism, offload weights to the CPU or the hard drive, and I've encountered some bugs linked to the way we use the `PreTrainedModel.device`: it grabs the first parameter of the model to infer a device for the whole model. This doesn't work when the model is:
- split on several devices and the first parameter grabbed happens to be on the wrong one
- not materialized because its parameters are offloaded on the CPU or the hard-drive.

So whenever it's possible, it would be great to rely on something else if we can, for instance some device where the inputs are. This PR does this for every use of this `device` attribute in modeling_utils and generation_utils, with the exception of some code where there are no inputs passed so we generate them and have to use something for the device.

If all works well, I plan to add all modeling files that make use of that attribute (when in the `dummy_inputs`, I'll leave the `self.device` but outside of it, will grab the device of any inputs we have).
_The documentation is not available anymore as the PR was closed or merged._