Implement Pad operator

[jeremiedb]

Following error resulted from attempt to load the Style Transfer ONNX model from:
https://github.com/onnx/models/blob/master/vision/style_transfer/fast_neural_style/models/opset8/mosaic.onnx

using Flux
using ONNX

julia> ONNX.load_model("mosaic.onnx")
ERROR: KeyError: key :Pad not found
Stacktrace:
 [1] getindex(::Dict{Symbol,Any}, ::Symbol) at .\dict.jl:478
 [2] _graph(::ONNX.Types.Graph) at C:\Users\Jeremie\.julia\packages\ONNX\iz0dL\src\graph\graph.jl:0
 [3] graph(::ONNX.Types.Graph) at C:\Users\Jeremie\.julia\packages\ONNX\iz0dL\src\graph\graph.jl:92
 [4] code(::ONNX.Types.Graph) at C:\Users\Jeremie\.julia\packages\ONNX\iz0dL\src\graph\graph.jl:100
 [5] write_julia_file(::String) at C:\Users\Jeremie\.julia\packages\ONNX\iz0dL\src\convert.jl:235
 [6] load_model(::String) at C:\Users\Jeremie\.julia\packages\ONNX\iz0dL\src\convert.jl:250
 [7] top-level scope at none:0