text: Title: Does `Trainer(devices=1)` use all CPUs?
  Body: ### Bug description
  
  https://github.com/Lightning-AI/pytorch-lightning/blob/3740546899aedad77c80db6b57f194e68c455e28/src/lightning/fabric/accelerators/cpu.py#L75
  
  `cpu_cores` being a list of integers will always raise an exception, which shouldn't according to the Trainer documentation/this function signature
  
  ### What version are you seeing the problem on?
  
  master
  
  ### How to reproduce the bug
  
  _No response_
  
  ### Error messages and logs
  
  ```
  # Error messages and logs here please
  ```
  
  
  ### Environment
  
  <details>
    <summary>Current environment</summary>
  
  ```
  #- Lightning Component (e.g. Trainer, LightningModule, LightningApp, LightningWork, LightningFlow):
  #- PyTorch Lightning Version (e.g., 1.5.0):
  #- Lightning App Version (e.g., 0.5.2):
  #- PyTorch Version (e.g., 2.0):
  #- Python version (e.g., 3.9):
  #- OS (e.g., Linux):
  #- CUDA/cuDNN version:
  #- GPU models and configuration:
  #- How you installed Lightning(`conda`, `pip`, source):
  #- Running environment of LightningApp (e.g. local, cloud):
  ```
  
  </details>
  
  
  ### More info
  
  _No response_
  
  cc @borda