* Function for retrieving number of node outputs.

* Handle optional trimmed outputs.

* Fix compilation err on clang.

* Fix error for number of outputs.

- Iterate over the minimum of number of outputs we return and the number
  of outputs of respective node in the graph. Some outputs may be
  optional and trimmed, as well as for some op implementations we may
  return not all outputs (ie. Dropout - where we do not return additional
  optional output).

* Update graph.cpp

* Add dropout ONNX op.

* Revert to iterate over node outputs in graph.

* Use more apropriate word in comment.