New method to set shape of model inputs

[fuzhenxin]

For tensorflow>=2.2.0, the model._set_inputs failed in

tflite-android-transformers/models_generation/gpt2.py

Line 7 in dcd6da1

model._set_inputs(input_spec, training=False)

, and the model.inputs and model.outpus are all None.
The following error will happen when running in Android:
java.lang.IllegalStateException: Internal error: Unexpected failure when preparing tensor allocations: tensorflow/lite/kernels/kernel_util.cc:249 d1 == d2 || d1 == 1 || d2 == 1 was not true.
For tensorflow<=2.1.0, the error will not happen.

The added code set inputs shape of model by adding an additional layer, and it works for both tensorflow>=2.2 and tensorflow<2.2

[jipson7]

Thank you. Can confirm this is working and necessary in TF >=2.2