class_docstr: Common tests for CachedDirectedFunction.
code: def inverse_keys(self):
      return self.cache.weak_keys(direction="inverse")
  
imports: from absl.testing import parameterized
  import tensorflow.compat.v2
  from tensorflow_probability.python.internal import cache_util
  from tensorflow_probability.python.internal import test_util
label_desc: Use image node_id 8 for calling the CacheTestBase obj's underlying member method code with example usage: obj.inverse_keys() and returns: self