title: TF 2.3 Concatenate error - AttributeError: 'Sequential' object has no attribute '_nested_outputs'
body: <p><code>fusion_model = tf.keras.layers.concatenate([m1.output, m2.output])</code></p>
  <p>m1 and m2 are Model objects.</p>
  <p>I'm concatenating the outputs of the two models and I'm getting the following error:</p>
  <p>AttributeError: 'Sequential' object has no attribute '_nested_outputs'</p>
  <p>This exact code used to work in TF 1.14. Any help? thanks</p>
  <p>After experimenting with this example, it appears to crash on the statement <code>m2.output</code> (<code>m1.output</code> runs fine)</p>
  <p>my M2 model is:</p>
  <pre><code>Layer (type)                 Output Shape              Param #   
  =================================================================
  rescaling_1 (Rescaling)      (None, 456, 456, 3)       0         
  _________________________________________________________________
  efficientnetb5 (Functional)  (None, 15, 15, 2048)      28513527  
  _________________________________________________________________
  global_average_pooling2d (Gl (None, 2048)              0         
  _________________________________________________________________
  dense_3 (Dense)              (None, 160)               327840    
  _________________________________________________________________
  dropout_19 (Dropout)         (None, 160)               0         
  _________________________________________________________________
  predictions (Dense)          (None, 19)                3059      
  </code></pre>