get rid of protobuf dependencies (#25650)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/25650
This PR removes protobuf dependencies from mobile build altogether:
- caffe2/proto: protobuf files, including caffe2.proto and torch.proto;
- caffe2 components that depend on caffe2.proto, including most part of
caffe2/core, caffe2/utils;
- libprotobuf / libprotobuf-lite dependencies;
- protobuf compiler;
- some utils class, e.g.: netdef_converter.cpp;
- introduce a macro to disable third_party/onnx which depends on protobuf;
Test Plan:
- builds;
- link with demo app to make sure it can load and run a model in pickle format;
Differential Revision: D17183548
Pulled By: ljk53
fbshipit-source-id: fe60b48674f29c4a9b58fd1cf8ece44191491531
