Fix OPT doc tests
# What does this PR do?
Fixes the doctest for OPT by adding slight modifications. 

_The documentation is not available anymore as the PR was closed or merged._<|||||>Hi @ArthurZucker Do you know why the expected value has extra ``` ? `OPTForCausalLM` has the same expected value, but it doesn't have test failure. Seems strange to me 😕 

```
 1003         >>> # Generate
 1004         >>> generate_ids = model.generate(inputs.input_ids, max_length=30)
 1005         >>> tokenizer.batch_decode(generate_ids, skip_special_tokens=True, clean_up_tokenization_spaces=False)[0]
 Expected:
     "Hey, are you consciours? Can you talk to me?\nI'm not consciours, but I can talk to you."
     ```
 Got:
     "Hey, are you consciours? Can you talk to me?\nI'm not consciours, but I can talk to you."
```<|||||>Actually it seems that the automatic doc préparation that adds a line does not affect the TFOPTForCausalLM forward doc. Thus I removed the manually created example and defeated to using **add_code_sample** which worked in that case. 
Now why does the add extra line does not work is another mystery and I could not setup a debugging instance to check that  <|||||>Thank you. I will check this on Monday.<|||||>Regarding the mystery , it is due to this line

> ``decoder_input_ids```

https://github.com/huggingface/transformers/blob/b2e4b091f08f1aaf21855d588c6c8d284baba9eb/src/transformers/models/opt/modeling_tf_opt.py#L601

~~We can fix this in this PR too :-) It should be simply~~

> \`decoder_input_ids\`<|||||>I forgot to change the name in the `add_docstring` will fix that
