Allow override of default tolerance for NgraphTestCase

[Py] Added gelu operator to Python API.

[ONNX] Update ONNX version to 1.5.0

Transformers don't need to export symbols.

Convert op/experimental to type_name descriptions

Switch core ops to type_name description.

* Convert CPU ops to new form

Remove obsolete sigmoid

* Fix export

* fix typo

* typo

[Py] Added clamp operator to Python API

* Convert PlaidML ops to new form

* style

* Add CF for Gather

* Style

* Move SlicePlan out of DynElimination, for reuse in ConstantFolding

* Add CF support for Slice

* Add CF for DynSlice

* Add <algorithm> to slice_plan.cpp, to make Windows happy