make tuple annotation more specific to avoid failures during symbolic_trace
This pr fixes the failure appeared during `symbolic_trace due` to the missing annotation of Tuple.

Before this pr, if we symbolic trace the bert model like:
```
from transformers import BertTokenizer, BertModel
from transformers.utils.fx import symbolic_trace

tokenizer = BertTokenizer.from_pretrained('bert-base-uncased')
model = BertModel.from_pretrained("bert-base-uncased")
text = "Replace me by any text you'd like."
encoded_input = tokenizer(text, return_tensors='pt')
output = model(**encoded_input)

symbolic_trace_model = symbolic_trace(model,  input_names=["input_ids", "attention_mask", "token_type_ids"])
```
We would get:
```
  exec(compile(src, key, 'exec'), globals)
  File "<eval_with_key>.0", line 4
    def forward(self, input_ids : typing_Union[torch.Tensor,NoneType], attention_mask : typing_Union[torch.Tensor,NoneType], token_type_ids : typing_Union[torch.Tensor,NoneType]) -> typing_Union[typing_Tuple[],transformers_modeling_outputs_BaseModelOutputWithPoolingAndCrossAttentions]:
                                                                                                                                                                                                                ^
SyntaxError: invalid syntax
```

After this pr, we could successfully symbolic trace the bert model.
_The documentation is not available anymore as the PR was closed or merged._<|||||>@chenbohua3 We're happy to merge this, please let us know whenever you're ready!<|||||>@Rocketknight1 Thank you very much. I'm also happy to merge this pr. Should I do anything else? Since this is my first pr to transformers repo:)<|||||>@chenbohua3 No, this looks good! We just like to check that contributors aren't planning to add anything else before we merge. Thank you for a great PR!