[T5, TF 2.2] change tf t5 argument naming
**Problem**: 
As shown in #3539, in TF 2.2 errors occur due to the naming of the first argument in the `keras.layer.__call__` function of TF T5.
Previously `decoder_input_ids` was used as the first argument - which did not produce any errors in TF <= 2.1. In TF 2.2, it produces the error: 

```
/usr/local/lib/python3.6/dist-packages/tensorflow/python/keras/engine/base_layer.py in __call__(self, *args, **kwargs)
    797     else:
    798       raise ValueError(
--> 799           'The first argument to `Layer.call` must always be passed.')
    800 
    801     call_context = base_layer_utils.call_context()

ValueError: The first argument to `Layer.call` must always be passed.
```

**Conclusion**
It seems that we have to change to a consistent naming, being `inputs` for the first argument of every `keras.layer.__call__` function