@Override
    public DataType setInputType(DataType inputType, TypeContext context) {
        if (inputType == null) return null;
        super.setInputType(inputType, context);
        if ( ! validType(inputType))
            throw new VerificationException(this, "Require a tensor with one dense dimension, but got " + inputType.getName());
        outputTensorType = outputType(((TensorDataType)inputType).getTensorType());
        return new TensorDataType(outputTensorType);
    }
Converts any tensor containing only ones and zeroes into one where each consecutive 8 values in the
dense dimension are packed into a single byte. As a consequence the output type of this is a tensor
where the dense dimension is 1/8th as large.

@author bratseth
indexinglanguage/src/main/java/com/yahoo/vespa/indexinglanguage/expressions/PackBitsExpression.java