@Override
    public DataType setOutputType(DataType outputType, TypeContext context) {
        super.setOutputType(outputType, context);
        if (outputType != null && ! validType(outputType))
            throw new VerificationException(this, "Required to produce " + outputType.getName() +
                                                  " but this produces a tensor with one dense dimension");
        outputTensorType = ((TensorDataType)outputType).getTensorType();
        return new TensorDataType(inputType(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