Better error for `torch::jit::load`ing a eager file (#31709)
Summary:
This adds a check to catch the case where someone `torch.save`s something then `torch::jit::load`s it in C++.
Relevant for #31620
](https://our.intern.facebook.com/intern/diff/19252172/)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/31709
Pulled By: driazati
Differential Revision: D19252172
fbshipit-source-id: f2a9b4442647285418b2778306629b4ff77c15e5
