onnxruntime

Fix bug in handling of an initializer that provides a graph output. (#3912)

* Outputs from model execution should always be returned in a newly allocated buffer or an pre-allocated buffer provided in fetches. When an initializer is providing a graph output (e.g. constant folding may result in this) we were returning an OrtValue that pointed to the initializer and not a separately allocated buffer with a copy.

This was wrong as:
  - value wasn't returned in a pre-allocated fetch so whilst the value returned was correct, it was returned in the wrong place
  - user could alter the data in the initializer via the returned value

* Add unit test with and without pre-allocated fetch.

* Add some extra info around why we're handling this special case.