A minor change in the `decoder_config` of T5Model
In the line mentioned below, 

https://github.com/huggingface/transformers/blob/fb3aa06cb673d0e2774a2924747d3290135c09cc/src/transformers/models/t5/modeling_t5.py#L1339,

I believe there should be an update in the `decoder_config`

The below is the change, I want to suggest

```python
decoder_config = copy.deepcopy(config) 
decoder_config.update("is_decoder", True)
```

This is because, we are using the decoder configuration, and when loading from the `PreTrained` class, the same is suggested.

Not sure whom to tag, @sgugger @NielsRogge, could you address it?


My mistake, sorry for the issue