Updated changelog with implemented features for 1.7.3 release (#2564)

Summary:
This PR adds the features that AbdelrahmanElmeniawy worked on during his internship and the speedups by alexanderguzhva in the CHANGELOG, ahead of the 1.7.3 release

Pull Request resolved: #2564

Reviewed By: algoriddle

Differential Revision: D41119343

Pulled By: mlomeli1

fbshipit-source-id: b41ce354440dea2a6f8f214bf6654ff453ef10e7

Author: mlomeli1

CHANGELOG.md

@@ -17,6 +17,16 @@ the Facebook Faiss team.  Feel free to add entries here if you submit a PR.
 - Support for fast accumulation of 4-bit LSQ and RQ
 - Added product additive quantization
 - Support per-query search parameters for many indexes + filtering by ids
+- write_VectorTransform and read_vectorTransform were added to the public API (by @AbdelrahmanElmeniawy)
+- Support for IDMap2 in index_factory by adding "IDMap2" to prefix or suffix of the input String (by @AbdelrahmanElmeniawy)
+- Support for merging all IndexFlatCodes descendants (by @AbdelrahmanElmeniawy)
+- Remove and merge features for IndexFastScan (by @AbdelrahmanElmeniawy)
+- Performance improvements: 1) specialized the AVX2 pieces of code speeding up certain hotspots, 2) specialized kernels for vector codecs (this can be found in faiss/cppcontrib)
+
+
+### Fixed
+- Fixed memory leak in OnDiskInvertedLists::do_mmap when the file is not closed (by @AbdelrahmanElmeniawy)
+- LSH correctly throws error for metric types other than METRIC_L2 (by @AbdelrahmanElmeniawy)
 
 ## [1.7.2] - 2021-12-15
 ### Added