text: Title: Arguments `lr_decay` and `lr_decay_steps` are not being used in MLP Model
  Body: ## 🐛 Bug Description
  
  Under [MLP](https://github.com/microsoft/qlib/blob/main/qlib/contrib/model/pytorch_nn.py) implementation there are several variables that need to be initialized. Such as `loss`, `lr`, `lr_decay`, `lr_decay_steps`, `optimizer`. However, it seems that the variables `lr_decay` and`lr_decay_steps` are indeed being initialized but are not being used at any point in the code.
  
  ## To Reproduce
  
  Steps to reproduce the behavior:
  
  1. Add under the file [workflow_config_mlp_Alpha360](https://github.com/microsoft/qlib/blob/main/examples/benchmarks/MLP/workflow_config_mlp_Alpha360.yaml) some not expected value into the variables `lr_decay` and`lr_decay_steps` such as `abc` and `efg`, respectively. Such as the example below
  
  ```
  model:
          class: DNNModelPytorch
          module_path: qlib.contrib.model.pytorch_nn
          kwargs:
              loss: mse
              lr: 0.002
              lr_decay: abc
              lr_decay_steps: def
              optimizer: adam
              max_steps: 8000
              batch_size: 4096
              GPU: 0
              pt_model_kwargs:
                input_dim: 360
  ```
  
  2. Execute `qrun examples/benchmarks/MLP/workflow_config_mlp_Alpha360_br.yaml`
  3. Then, it should execute successfully, which is an akward behavior since the variables `lr_decay` and`lr_decay_steps` should be numbers. **NOTE**, the problem is not with the fact that such variables are not being checked to validate its types before initilization, but with the fact that such variables are not being referenced at any time in the code. 
  
  
  ## Expected Behavior
  
  If the variables `lr_decay` and`lr_decay_steps` were being used and the user passed invalid values it should throw an error.
  
  ## Screenshot
  
  No need.
  
  ## Environment
  
  **Note**: User could run `cd scripts && python collect_info.py all` under project directory to get system information
  and paste them here directly.
  
  ```
  Linux
  x86_64
  Linux-5.13.0-39-generic-x86_64-with-glibc2.17
  #44~20.04.1-Ubuntu SMP Thu Mar 24 16:43:35 UTC 2022
  
  Python version: 3.8.13 (default, Mar 28 2022, 11:38:47)  [GCC 7.5.0]
  
  Qlib version: 0.8.4.99
  numpy==1.22.3
  pandas==1.4.2
  scipy==1.8.0
  requests==2.27.1
  sacred==0.8.2
  python-socketio==5.5.2
  redis==4.2.2
  python-redis-lock==3.7.0
  schedule==1.1.0
  cvxpy==1.2.0
  hyperopt==0.1.2
  fire==0.4.0
  statsmodels==0.13.2
  xlrd==2.0.1
  plotly==5.7.0
  matplotlib==3.5.1
  tables==3.7.0
  pyyaml==6.0
  mlflow==1.24.0
  tqdm==4.64.0
  loguru==0.6.0
  lightgbm==3.3.2
  tornado==6.1
  joblib==1.1.0
  fire==0.4.0
  ruamel.yaml==0.17.21
  ```
  
  ## Additional Notes
  
  No need.
  