class_docstr: Common tests for CachedDirectedFunction.
code: def forward_impl(self, x, **kwds):
      raise NotImplementedError()
  
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 2 for calling the CacheTestBase obj's underlying member method code with example usage: obj.forward_impl(x) without return types