onnxruntime

Allow multiple sessions to share an allocator, optimize constant folding memory usage, expose arena configs. (#4813)

* Add support for sharing allocators

* Incremental update

* Address some PR comments, add unit tests, add documentation.

* Address PR comments, add tests and some documentation.

* Fix build and test issues

* Remove RegisterAllocator API restoring the OrtAllocator interface changes. Changed docs to reflect this.
Also fixed the orttraining segfault. The segfault was because in the case of training session,
the CPU exec prov is not available at the time the transformers are applied. Changed it to create
a new one.