[Frontend][Keras] Support nested layers recursively in keras frontend

[trevor-m]

Keras models can have nested layers, where one layer in the model is actually another Keras model with its own layers. With this PR, we will now support these types of models in TVM.

Example Keras model with nested model "mobilenetv2_1.00_224".

Layer (type)                 Output Shape              Param #   
=================================================================
mobilenetv2_1.00_224 (Functi (None, 7, 7, 1280)        2257984   
_________________________________________________________________
global_average_pooling2d_1 ( (None, 1280)              0         
_________________________________________________________________
dense_2 (Dense)              (None, 1024)              1311744   
_________________________________________________________________
dense_3 (Dense)              (None, 2)                 2050      
=================================================================
Total params: 3,571,778
Trainable params: 3,537,666
Non-trainable params: 34,112
_________________________________________________________________

It can be created using the following Keras code, which is commonly used when using pretained models.

sub_model = keras.applications.MobileNetV2(input_shape=(224, 224, 3), include_top=False)
keras_model = keras.Sequential(
  [
    sub_model,
    keras.layers.GlobalAveragePooling2D(),
    keras.layers.Dense(1024),
    keras.layers.Dense(2),
  ]
)

[trevor-m]

@siju-samuel @jwfromm @mbrookhart @leandron Could you please review?

[leandron]

LGTM

[mbrookhart]

LGTM

[trevor-m]

Thanks for reviewing @mbrookhart @leandron !
Tests are passing

[mbrookhart]

Thanks @trevor-m @leandron