Function extract_corpus_parallel 

pub fn extract_corpus_parallel<F, C>(
    files: &[PathBuf],
    config: CorpusParallelConfig,
    extractor: F,
    on_chunk_done: Option<C>,
) -> ChunkResult
where
    F: Fn(&[PathBuf]) -> Result<ChunkResult, String> + Sync + Send + 'static,
    C: Fn(usize, usize, &ChunkResult) + Send + Sync + 'static,