* Concat operator is updated to support dynamic_shape
1) Added new concat_negative_indexing test
2) Replaced size_t -> int64_t
3) Support for a negative indexing, calculate axis = axis + int64(input_rank)
   if (axis < 0) { axis = axis + int64_t(this_input_rank); }

* Remove unwanted #include "ngraph/op/constant.hpp" header

* Refactoring:
1) The name of variable is replaced: m_concatenation_axis -> m_axis

* Concat negative indexing test is adjusted to support dynamic_shape tensor
auto pshape_a = PartialShape::dynamic(); for each tensor {a,b,c} result tensor has dynamic_shape

* The backend supports dynamic shapes
auto backend = runtime::Backend::create("${BACKEND_NAME}", true);

* Other shape is supported by concat:
set_output_type(0, inputs_et, PartialShape::dynamic(concatenation_axis_output_dim));

* The NODE_VALIDATION_CHECK was moved up to be for a dynamic_shape

* [Test] The shape of output tensor was changed
[Concat CPU] Added support for a negative indexing on a cpu

* Review changes:
1) Added axis re-calculate for a reference version of concat
2) axis is not replied

* Review changes: support for a negative axis

* Comment about variable is added to concat.hpp file
Removed unused variable