class_docstr: Cache tests for bijectors that don't return new things.
code: def forward_impl(self, struct):
      x, y = struct
      return y, x
  
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 1 for calling the IdentityCacheTest obj's underlying member method code with example usage: obj.forward_impl(struct) and returns: y,  x