Set KnnDescriptor.desc_name in the Benchmarking core framework in FAISS like other descriptors (facebookresearch#4109)

Summary:
Pull Request resolved: facebookresearch#4109

Set KnnDescriptor.desc_name in the Benchmarking core framework in FAISS like other descriptors

Reviewed By: mnorris11

Differential Revision: D67539874

fbshipit-source-id: 09ffb76296f466ae2d3b0eb551917f429bc7300f

Author: satymish

benchs/bench_fw/descriptors.py

@@ -341,6 +341,8 @@ def __hash__(self):
         return hash(str(self))
 
     def get_name(self):
+        if self.desc_name is not None:
+            return self.desc_name
         name = self.index_desc.get_name()
         name += IndexBaseDescriptor.param_dict_to_name(self.search_params)
         name += self.query_dataset.get_filename(KnnDescriptor.FILENAME_PREFIX)
@@ -350,6 +352,7 @@ def get_name(self):
             name += "rec."
         else:
             name += "knn."
+        self.desc_name = name
         return name
 
     def flat_name(self):