Change how caching/cleanup for CapturedTraceback works (#107471)
CapturedTraceback is fast but one downside is that it has strong references to code objects, which via `co_extra` can cause un-collectable cycles. This means that it is important to clear out CapturedTraceback when you are done with it; e.g., if you tracebacks during compilation, you need to explicitly clear them out at the end of compilation to actually make sure they promptly deallocate.
Instead of caching `summary` on the CapturedTraceback, we simply allow for tracebacks to have `tb = None`. Tracebacks get dropped if you pickle the traceback, or if you explicitly call cleanup().
Signed-off-by: Edward Z. Yang <ezyang@meta.com>
Pull Request resolved: https://github.com/pytorch/pytorch/pull/107471
Approved by: https://github.com/voznesenskym
ghstack dependencies: #107505, #107516, #107530, #107532, #107562
