Add tutorial for FastScan (#3465)

Summary:
Pull Request resolved: #3465

This commit include python version of tutorial for FastScan. It includes all the parameters enabled within PQFastScan.

Reviewed By: junjieqi

Differential Revision: D57594044

fbshipit-source-id: cb12679b6fc241a654b9545c5bc7bd0517aa1813

Author: Xiao Fu

tutorial/python/7-PQFastScan.py

@@ -0,0 +1,35 @@
+# Copyright (c) Facebook, Inc. and its affiliates.
+#
+# This source code is licensed under the MIT license found in the
+# LICENSE file in the root directory of this source tree.
+
+import faiss
+import numpy as np
+
+d = 64                           # dimension
+nb = 100000                      # database size
+nq = 10000                       # nb of queries
+np.random.seed(1234)             # make reproducible
+xb = np.random.random((nb, d)).astype('float32')    # 64-dim *nb queries
+xb[:, 0] += np.arange(nb) / 1000.
+xq = np.random.random((nq, d)).astype('float32')
+xq[:, 0] += np.arange(nq) / 1000.
+
+m = 8   # 8 specifies that the number of sub-vector is 8
+k = 4   # number of dimension in etracted vector
+n_bit = 4   # 4 specifies that each sub-vector is encoded as 4 bits
+bbs = 32    # build block size ( bbs % 32 == 0 ) for PQ
+index = faiss.IndexPQFastScan(d, m, n_bit, faiss.METRIC_L2, bbs)
+# construct FastScan Index
+
+assert not index.is_trained
+index.train(xb)     # Train vectors data index within mockup database
+assert index.is_trained
+
+index.add(xb)
+D, I = index.search(xb[:5], k)  # sanity check
+print(I)
+print(D)
+index.nprobe = 10              # make comparable with experiment above
+D, I = index.search(xq, k)     # search
+print(I[-5:])               # neighbors of the 5 last queries