PretrainedModel: made `_load_pretrained_model_low_mem` static + bug fix
# What does this PR do?
This PR makes `PretrainedModel._load_pretrained_model_low_mem` a `staticmethod` since it's not returning an instance of `PretrainedModel`. I've also fixed a bug inside the method itself, the main for loop was wrong.
_The documentation is not available anymore as the PR was closed or merged._<|||||>Merging this, since `low_cpu_mem_usage` is not working on master right now and is required for to test the addition of some new models.

@stas feel free to still leave a comment about the `classmethod` change :) 