Add support for stride in Slice operator

[mciprian13]

Add stride parameter in the Slice operator. The stride parameter should be allowed to be both positive and negative (slicing from the end). Both ONNX and TensorFlowLite formats define Slice operators using this parameter.

[Lewuathe]

@mciprian13 I could not find stride parameter in the Slice operator of ONNX. Do you mean by that to support the negative index in starts and ends?

[mciprian13]

@Lewuathe It's the steps parameter from the the Slice operator of ONNX (Stride/step means the same thing).