Add warning regarding pickle insecurity on torch.load documentation (#32593)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/31875
Added a small warning box based on the one presented on the [pickle](https://docs.python.org/3/library/pickle.html) module regarding the safety issues of unpickling files. i.e., unwanted code execution.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/32593
Differential Revision: D19572292
Pulled By: ngimel
fbshipit-source-id: 69e7de390133ea77bddcadcd5b6820193c8abcc9
