Doc operators

Context

This MR is dedicated to add documentation for Operators.

Resolves issue #217

TODO

• Abs
• Add
• And
• ArgMax
• Atan
• AvgPooling
• BatchNorm
• BitShift
• Cast
• Clip
• Concat
• ConstantOfShape
• Conv
• ConvDepthWise
• DepthToSpace
• Div
• Erf
• FC
• Flatten
• Fold
• Gather
• GenericOperator
• GlobalAveragePooling
• GridSample
• Heaviside
• Identity
• ILayerNorm
• LeakyReLU
• Ln
• LRN
• MatMul
• MaxPooling
• Memorize
• MetaOperator
• Move
• Mul
• Operator
• OperatorTensor
• Pad
• Pop
• Pow
• Producer
• ReduceMean
• ReduceSum
• ReLU
• Reshape
• Resize
• Round
• Scaling
• Shape
• ShiftGELU
• ShiftMax
• Sigmoid
• Slice
• Softmax
• Split
• Sqrt
• Squeeze
• Stack
• Sub
• Tanh
• Transpose
• Unfold
• Unsqueeze
• Weighterleaving