

TITLE
Model inputs and outputs are ``None`` when converting fine-tuned gpt2 to Tensorflow?

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Hi,

I've fine tuned a distilgpt2 model using my own text using ``run_language_modeling.py`` and its working fine after training and ``run_generation.py`` script produces the expected results.

Now I want to convert this to a Tensorflow Lite model and did so by using the following 

```python
from transformers import *

CHECKPOINT_PATH = '/content/drive/My Drive/gpt2_finetuned_models/checkpoint-2500'

model = GPT2LMHeadModel.from_pretrained("distilgpt2")
model.save_pretrained(CHECKPOINT_PATH)
model = TFGPT2LMHeadModel.from_pretrained(CHECKPOINT_PATH, from_pt=True) 
```

But I dont think I'm doing this right as after conversion, when I write

```python
print(model.inputs)
print(model.outputs)
```

I get 

```
None
None
```

But I still went ahead with the TFLite conversion using :

```python
import tensorflow as tf

input_spec = tf.TensorSpec([1, 64], tf.int32)
model._set_inputs(input_spec, training=False)

converter = tf.lite.TFLiteConverter.from_keras_model(model)

# FP16 quantization:
converter.optimizations = [tf.lite.Optimize.DEFAULT]
converter.target_spec.supported_types = [tf.float16]

tflite_model = converter.convert()

open("/content/gpt2-fp16.tflite", "wb").write(tflite_model)
```

But does not work and when using the generated ``tflite`` model I get the error:

> tensorflow/lite/kernels/kernel_util.cc:249 d1 == d2 || d1 == 1 || d2 == 1 was not true.

Which I'm sure has something to to with my model not converting properly and getting ``None`` for input/output.


Does anyone have any idea how to fix this?

Thanks