Fitting hangs at "cleaning up ddp environment..." when tpu_cores=8
🐛 Bug
When setting tpu_cores of Trainer to 8, fitting hangs at "cleaning up ddp environment...".
Please reproduce using the BoringModel
https://colab.research.google.com/drive/1tJswNaT0I-GrGsi6ngwwRDUmeFY1pFr3?usp=sharing
To Reproduce
Run above URL notebook.
Expected behavior
Trainer.fit ends normally .
Environment

PyTorch Version (e.g., 1.0): 1.7.0a0+7e71a98
OS (e.g., Linux): Linux
How you installed PyTorch (conda, pip, source): pip
Build command you used (if compiling from source): N/A
Python version: 3.7.10
CUDA/cuDNN version: N/A
GPU models and configuration: N/A
Any other relevant information: I tried the notebook on Google Colab with TPU.