QA doc: import torch before it is used (#24228)

* import torch before it is used

* style
Signed-off-by: byhsu <byhsu@linkedin.com>

---------
Signed-off-by: byhsu <byhsu@linkedin.com>
Co-authored-by: byhsu <byhsu@linkedin.com>

docs/source/en/tasks/question_answering.mdx

@@ -369,6 +369,7 @@ Tokenize the text and return PyTorch tensors:
 Pass your inputs to the model and return the `logits`:
 
 ```py
+>>> import torch
 >>> from transformers import AutoModelForQuestionAnswering
 
 >>> model = AutoModelForQuestionAnswering.from_pretrained("my_awesome_qa_model")