Undefined name: import csr_matrix for line 166

`csr_matrix()` is called on line 166 but it is never defined or imported.
https://docs.scipy.org/doc/scipy/reference/generated/scipy.sparse.csr_matrix.html

openselfsup/third_party/clustering.py

@@ -5,6 +5,7 @@ import time
 import numpy as np
 import faiss
 import torch
+from scipy.sparse import csr_matrix
 
 __all__ = ['Kmeans', 'PIC']