Fix Slice, MatMul, and ConstantOfShape in ONNX importer

[TomonobuTsujikawa]

This PR fixes the following issues in ONNX importer.

• normalize_shape()
  • If its argument type is RepeatedScalarContainer, returned value is not normalized.
• Slice
  • output shape in Slice conversion may become negative.
• MatMul
  • The existing implementation only supports the case that input tensors have the same number of dimensions.
  • This PR implements full support of ONNX MatMul.
• ConstantOfShape
  • The existing implementation only does not support raw_data in TensorProto.