[pytorch] move legacy deserialization code into jit/import_legacy.cpp

[ljk53]

Stack from ghstack:

• [pytorch][cmake] clean up binaries/cmake for mobile #25651 [pytorch][cmake] clean up binaries/cmake for mobile
• [pytorch][cmake] get rid of protobuf dependencies #25650 [pytorch][cmake] get rid of protobuf dependencies
• [pytorch] move GetDimFromOrderString to caffe2/core/types.h #25671 [pytorch] move GetDimFromOrderString to caffe2/core/types.h
• [pytorch] remove caffe2.pb.h dependency for embedding_lookup_idx.cc #25670 [pytorch] remove caffe2.pb.h dependency for embedding_lookup_idx.cc
• [pytorch] move legacy deserialization code into jit/import_legacy.cpp #25649 [pytorch] move legacy deserialization code into jit/import_legacy.cpp

Summary:
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

[zdevito]

Looks good to me. Thanks for the cleanup!

[facebook-github-bot]

@ljk53 merged this pull request in a35a63b.