[docs] Properly link register_post_accumulate_grad_hook docs (#108157)
it shows up now
![image](https://github.com/pytorch/pytorch/assets/31798555/0aa86839-b9c5-4b4b-b1b1-aa1c0c0abbab)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/108157
Approved by: https://github.com/soulitzer, https://github.com/albanD