text: Title: Upgraded from version 4.0.1. to 4.2.0, now KeyedVectors.most_similar() throws error
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  #### Problem description
  
  Using KeyedVectors.most_similar() as follows:
  
  learn_language.most_similar(positive=[class_vectors[classname]], negative=[class_vectors[aclassname]], topn=max_topn)
  
  where class_vectors[X] are some vectors created by averaging a number of KeyedVectors (of the size 75 or shape (75,)) from the same KeyedVectors object.
  
  WORKS perfectly well in version 4.0.1, but on version 4.2.0 throws the following error:
  
  learn_language.most_similar(positive=[class_vectors[classname]], negative=[class_vectors[aclassname]], topn=max_topn)
    File "/usr/local/lib/python3.9/site-packages/gensim/models/keyedvectors.py", line 842, in most_similar
      mean = self.get_mean_vector(keys, weight, pre_normalize=True, post_normalize=True, ignore_missing=False)
    File "/usr/local/lib/python3.9/site-packages/gensim/models/keyedvectors.py", line 512, in get_mean_vector
      mean += weights[idx] * key
  ValueError: operands could not be broadcast together with shapes (100,) (75,) (100,) 
  
  #### Steps/code/corpus to reproduce
  
  ```python
  import gensim
  import numpy as np
  learn_language = gensim.models.KeyedVectors.load("embeddings.english", mmap="r")
  learn_language.most_similar(positive=[learn_language["great"]], negative=[learn_language["terrible"]], topn=100)
  
  learn_language.most_similar(positive=[learn_language["great"]], negative=[learn_language["terrible"]], topn=100)
    File "/usr/local/lib/python3.9/site-packages/gensim/models/keyedvectors.py", line 842, in most_similar
      mean = self.get_mean_vector(keys, weight, pre_normalize=True, post_normalize=True, ignore_missing=False)
    File "/usr/local/lib/python3.9/site-packages/gensim/models/keyedvectors.py", line 512, in get_mean_vector
      mean += weights[idx] * key
  ValueError: operands could not be broadcast together with shapes (100,) (75,) (100,) 
  ```
  
  #### Versions
  
  >>> import platform; print(platform.platform())
  Windows-10-10.0.19041-SP0
  >>> import sys; print("Python", sys.version)
  Python 3.9.4 (tags/v3.9.4:1f2e308, Apr  6 2021, 13:40:21) [MSC v.1928 64 bit (AMD64)]
  >>> import struct; print("Bits", 8 * struct.calcsize("P"))
  Bits 64
  >>> import numpy; print("NumPy", numpy.__version__)
  NumPy 1.22.4
  >>> import scipy; print("SciPy", scipy.__version__)
  SciPy 1.8.1
  >>> import gensim; print("gensim", gensim.__version__)
  gensim 4.2.0
  >>> from gensim.models import word2vec;print("FAST_VERSION", word2vec.FAST_VERSION)
  FAST_VERSION 0