move legacy deserialization code into jit/import_legacy.cpp (#25649)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/25649
Continue the work of PR #25493 to remove dependencies of generated
protobuf headers from jit/import.cpp.
Instead of adding intrusive #if/#else to gate the legacy functions,
moving them into a separate file. Keep the ScriptModuleDeserializer
structure as otherwise it will require a lot of interface changes.
There is not much state to copy from ScriptModuleDeserializer as it only
extracts extra_files before calling into LEGACY_deserialize. There is
no state to copy back into ScriptModuleDeserializer either as it directly
returns script::Module.
Test Plan:
- builds;
- with stacked PR to remove protobuf from cmake;
- load and run ResNet-18 in model.json format with non-mobile build;
- load and run ResNet-18 in pickle format with mobile build;
Differential Revision: D17183549
Pulled By: ljk53
fbshipit-source-id: 2947b95659cd16046d9595fb118d22acc179b3ad
