Update pgvector loading method to use binary format (#488)

Particularly on large vector types, the pgvector module was spending
significant time on converting floating point values to ASCII before
being transmitted to the PostgreSQL server. This changes keeps the
format in binary, reducing overhead. One test demonstrated a 63%
reduction in load time, which would have an impact on the overall
"build" time as reported by this benchmark.

ann_benchmarks/algorithms/pgvector/module.py

@@ -30,7 +30,8 @@ def fit(self, X):
         cur.execute("CREATE TABLE items (id int, embedding vector(%d))" % X.shape[1])
         cur.execute("ALTER TABLE items ALTER COLUMN embedding SET STORAGE PLAIN")
         print("copying data...")
-        with cur.copy("COPY items (id, embedding) FROM STDIN") as copy:
+        with cur.copy("COPY items (id, embedding) FROM STDIN WITH (FORMAT BINARY)") as copy:
+            copy.set_types(["int4", "vector"])
             for i, embedding in enumerate(X):
                 copy.write_row((i, embedding))
         print("creating index...")