🐛 Saving TF model : Expected Operation, Variable, or Tensor, got None
# 🐛 Bug

I'm training TFElectraModel. Training goes well, but when I try to save the model, I met the following error :

`TypeError: Expected Operation, Variable, or Tensor, got None`

## Minimal reproducible example

```
!pip install transformers

from transformers import TFElectraModel

model = TFElectraModel.from_pretrained("google/electra-base-discriminator")
!mkdir swag
model.save("swag")
```

## Related

#2336
https://github.com/tensorflow/tensorflow/issues/35432

Hi! We prefer using our own API for this, using `from_pretrained` which also takes care of the configuration:

```py
from transformers import TFElectraModel

model = TFElectraModel.from_pretrained("google/electra-base-discriminator")
!mkdir swag
model.save_pretrained("swag")
```

Is there a reason why you would rather use `.save` rather than `.save_pretrained`?<|||||>I was using `.save()` because I made a custom model, with additional layers on top of Electra, and I want to save these layers as well.

Finally, I simply sub-classed `TFElectraPreTrainedModel` and add my custom layers there, so I can use `.save_pretrained`.

Thanks for the help !<|||||>I am also facing similar issue.
The reason for using save method is i want to host the saved model using tensorflow model server<|||||>@nirajkale did you manage to solve this? I'm facing the same issue with other pre-trained.