Fix linear decay for learning rate in Doc2Vec.infer_vector. Fix #2061…

… (#2063)

* Linear decay for learning rate in infer_vector

* remove whitespace

gensim/models/doc2vec.py

@@ -546,6 +546,8 @@ def infer_vector(self, doc_words, alpha=0.1, min_alpha=0.0001, steps=5):
         if not self.sg:
             neu1 = matutils.zeros_aligned(self.trainables.layer1_size, dtype=REAL)
 
+        alpha_delta = (alpha - min_alpha) / (steps - 1)
+
         for i in range(steps):
             if self.sg:
                 train_document_dbow(
@@ -562,7 +564,7 @@ def infer_vector(self, doc_words, alpha=0.1, min_alpha=0.0001, steps=5):
                     self, doc_words, doctag_indexes, alpha, work, neu1,
                     learn_words=False, learn_hidden=False, doctag_vectors=doctag_vectors, doctag_locks=doctag_locks
                 )
-            alpha = ((alpha - min_alpha) / (steps - i)) + min_alpha
+            alpha -= alpha_delta
 
         return doctag_vectors[0]