Large model loading: add link to existing documentation
The documentation for large model loading is in two different places. This adds a link from one to the other, showing the auto device map.
_The documentation is not available anymore as the PR was closed or merged._<|||||>Hmmm yes, I agree, but this is still linking to `from_pretrained`, just a part of the documentation that contains more information about the `low_cpu_mem_usage`. The part I have removed glosses over it quickly, whereas the part I link to has extensive documentation covering both `low_cpu_mem_usage` and the `device_map` argument to pass to `from_pretrained`.

Reading the documentation right now, if we're interested in big models and we open the "Instantiating a big model" page in the toctree, there are no mention of the `device_map`. This is what this PR aims to fix.