Account for inference mode in FakeTensor cache (#119963)
Summary: an fbcode test exposed a shortcoming where we serve a FakeTensor from the cache with the wrong inference_mode. Take the current mode into account in the cache key so we only serve entries from the same mode we're in currently
Test Plan: New unit test
Pull Request resolved: https://github.com/pytorch/pytorch/pull/119963
Approved by: https://github.com/eellison
