code: def forward(self, inputs, attn_bias=None, past_cache=None):
      if past_cache is not None:
          assert isinstance(past_cache, tuple), (
              "unknown type of `past_cache`, expect tuple or list. got %s"
              % repr(type(past_cache))
          )
          past_cache = list(zip(*past_cache))
      else:
          past_cache = [None] * len(self.block)
      cache_list_k, cache_list_v, hidden_list = [], [], [inputs]
  
      for b, p in zip(self.block, past_cache):
          inputs, cache = b(inputs, attn_bias=attn_bias, past_cache=p)
          cache_k, cache_v = cache
          cache_list_k.append(cache_k)
          cache_list_v.append(cache_v)
          hidden_list.append(inputs)
  
      return inputs, hidden_list, (cache_list_k, cache_list_v)
  
imports: from __future__ import division
  from __future__ import absolute_import
  from __future__ import print_function
  from __future__ import unicode_literals
  import logging
  import paddle.fluid.dygraph
  import paddle.fluid
  import paddle.fluid.layers
label_desc: Use image node_id 2 for calling the ErnieEncoderStack obj's underlying member method code with example usage: obj.forward(inputs, attn_bias, past_cache) and returns: inputs,  hidden_list,  cache_list_k,  cache_list_v