Fix the error: 'SSDataManager' object has no attribute

[SimFG]

issue: #489

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[codecov]

Codecov Report

Merging #501 (8709688) into dev (635dca5) will not change coverage.
The diff coverage is 100.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev     #501   +/-   ##
=======================================
  Coverage   94.19%   94.19%           
=======================================
  Files          94       94           
  Lines        3908     3908           
=======================================
  Hits         3681     3681           
  Misses        227      227           

Files Changed	Coverage Δ
gptcache/manager/data_manager.py	89.50% <100.00%> (ø)