Fix CI error of PyTorch

[nzw0301]

Expected behavior

CIs should be passed without error.

Environment

This repo's Github CI: https://github.com/optuna/optuna-examples/actions/workflows/pytorch.yml

Error messages, stack traces, or logs

Error message is as follows:

Run python pytorch/pytorch_lightning_simple.py
Traceback (most recent call last):
  File "pytorch/pytorch_lightning_simple.py", line 19, in <module>
    from optuna.integration import PyTorchLightningPruningCallback
ImportError: cannot import name 'PyTorchLightningPruningCallback' from 'optuna.integration' (unknown location)
Error: Process completed with exit code 1.

Steps to reproduce

See https://github.com/optuna/optuna-examples/actions/workflows/pytorch.yml

Additional context (optional)

1. Optuna's main repo doesn't see this CI error
2. ubuntu-latest has been released after this CI had been broken, the version of ubuntu were same:

From the first failed CI's log

Current runner version: '2.286.0'
Operating System
Virtual Environment
  Environment: ubuntu-20.04
  Version: 20211219.1
  Included Software: https://github.com/actions/virtual-environments/blob/ubuntu20/20211219.1/images/linux/Ubuntu2004-README.md
  Image Release: https://github.com/actions/virtual-environments/releases/tag/ubuntu20%2F20211219.1

From the last complete CI's log

Current runner version: '2.285.1'
Operating System
Virtual Environment
  Environment: ubuntu-20.04
  Version: 20211219.1
  Included Software: https://github.com/actions/virtual-environments/blob/ubuntu20/20211219.1/images/linux/Ubuntu2004-README.md
  Image Release: https://github.com/actions/virtual-environments/releases/tag/ubuntu20%2F20211219.1

3. the current runner versions were not the same between the two messages above, but I'm not sure that causes CI error...

[nzw0301]

The error does hot happen anymore on the main branch...
https://github.com/optuna/optuna-examples/actions/runs/1736220267

Let me close this issue once.