*   Add `sparse_column_with_vocabulary_file`, to specify a feature column that
    transform string features to IDs, where the mapping is defined by a
    vocabulary file.
*   Added `index_to_string_table` which returns a lookup table that maps indices
    to strings.
*   Add `string_to_index_table`, which returns a lookup table that matches
    strings to indices.
*   Add a `ParallelForWithWorkerId` function.