Summary:
Our strategy for detecting fake tensor leakage in non-strict for outside scope (side effects happening outside of model.forward) is:
1. We do gc.collect() before export and get the alive fake tensors
2. We dump the proxy to fake tensor map from make_fx tracer
3. We query gc again to get alive fake tensors
4. We take the delta between (1) and (3)
5. Filter out fake tensors that are:
1. Associated with `TrackedFake` (input tracking thing in symbolic_shapes)
2. Associated with `gm.meta`
6. Do ID match with the proxies and emit their stacktraces.
We rely on (https://github.com/pytorch/pytorch/pull/159923) for other sources of leakages such as:
1. We failed to proxy an operator (like param.data)
2. We cache some tensor in model.forward (https://github.com/pytorch/pytorch/issues/155114)
In general, we notice `gc.collect()` and query-ing gc for live objects are kinda slow. So we turn on this feature under env variable. We should document on export public facing documents that if you run into weird errors regarding fake tensors, they should look into turning on this env variable for further analysis.
Test Plan:
Test plan
Rollback Plan:
Differential Revision: D80003204
Pull Request resolved: https://github.com/pytorch/pytorch/pull/160456
Approved by: https://github.com/pianpwk
