Test the default pylucene bm25 for open-domain QA (Natural Questions)

Only tested for java-8-openjdk-amd64

export JAVA_HOME=/usr/lib/jvm/java-8-openjdk-amd64

Construct the corpus index

python indexer.py --corpus-path /data/xwhan/data/nq-dpr/corpus/psgs_w100.tsv --index-path /home/xwhan/data/nq-dpr/corpus/lucene_index

python retriever.py

Answer Recall

Method	Top20	Top 50	Top 100
pylucene	61.9	71.0	76.5
DPR	78.4	-	85.4