Evaluate, ngraph_reader check and Python nGraph API for GatherElements (#3624)

* add GatherElements evaluate to interpreter backend

* Finally successfully run on backend

* debugged non-typical cases

* added ngraph_reader tests for GatherElements

* added Python API for GatherElements

* apply_style

* corrected python api tests

* applied comments

* style-apply

* finally corrected nGraph Python API for GatherElements

* minor corrections

* style-apply

* replaced quotes

* added blank line

* corrected evaluate and disabled unit-tests for not yet supported plugins

* style-apply

* applied comments: negative tests added and additional checks in evaluate

* added bound check for axis in evaluate

* style-apply

* apply review comments

* fast correct evaluate for GatherElements

* style-apply

* revert changes in interpreter unit_test.manifest for Gather

* 🚀 optimized general solution; added separate calculation for 2D

* 🚀 applied comments

* style-apply

ngraph/python/setup.py

@@ -45,6 +45,7 @@ packages = [
     "ngraph.opset3",
     "ngraph.opset4",
     "ngraph.opset5",
+    "ngraph.opset6",
     "ngraph.utils",
     "ngraph.impl",
     "ngraph.impl.op",