PPO2: Call total_episode_reward_logger before incrementing num_timesteps

[balintkozma]

Description

In ppo2.py total_episode_reward_logger should be called before incrementing num_timesteps. See explanation in #143.

Motivation and Context

It improves the reported episode reward logging issue, and a PR was asked.

• [x] I have raised an issue to propose this change (required for new features and bug fixes)

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] Documentation (update in the documentation)

Checklist:

• [x] I've read the CONTRIBUTION guide (required)
• [x] I have updated the changelog accordingly (required).
• [ ] My change requires a change to the documentation.
• [ ] I have updated the tests accordingly (required for a bug fix or a new feature).
• [ ] I have updated the documentation accordingly.

[araffin]

Hello,

Please fill in the PR template completely.