text: Title: parameter alpha in focal loss is not the same as that of paper
  Body: ### Describe the bug
  
  The parameter alpha in focal loss is a scalor, so it couldn't has the effect of balanced the different classes. I think alpha should be a  tensor with n_classses element and every value refers to the weiht of this class
  
  ### Reproduction steps
  
  ```bash
  Please refer to the description
  ```
  
  
  ### Expected behavior
  
  Please refer to the description
  
  ### Environment
  
  ```shell
  wget https://raw.githubusercontent.com/pytorch/pytorch/main/torch/utils/collect_env.py
  # For security purposes, please check the contents of collect_env.py before running it.
  python collect_env.py
  ```
  - PyTorch Version (e.g., 1.0):
  - OS (e.g., Linux):
  - How you installed PyTorch (`conda`, `pip`, source):
  - Build command you used (if compiling from source):
  - Python version:
  - CUDA/cuDNN version:
  - GPU models and configuration:
  - Any other relevant information:
  ```
  
  
  ### Additional context
  
  _No response_