onnxruntime

Flatbuffers schema for serialization of the onnxruntime::model/graph (#4870)

* add flatbuffers submodule

* test version of flat buffer schema

* test version of flat buffer schema

* minor updates

* add serialization of the value info, group defs in different namespace

* update comments

* update cgmanifest.json

* update namespace, changed typeinfovalue to use union, added root_type and file_identifier

* add new container type, add max_node_index to graph

* add serializing session state

* addressed review comments

* minor updates

Co-authored-by: gwang0000 <62914304+gwang0000@users.noreply.github.com>