Add a callback to log raw stats

[vwxyzjn]

This PR adds a callback to log raw (unaveraged) stats such as episodic_length, episodic_returns. The testing command is python train.py --algo ppo --env CartPole-v1 -params callback:'"utils.callbacks.RawStatisticsCallback"' --track and it produces this run

Motivation and Context

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

Types of changes

• 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:

• I've read the CONTRIBUTION guide (required)
• 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.
• I have reformatted the code using make format (required)
• I have checked the codestyle using make check-codestyle and make lint (required)
• I have ensured make pytest and make type both pass. (required)

Note: we are using a maximum length of 127 characters per line

[vwxyzjn]

@araffin also suggested I do the same for EvalCallback. So I see that ExperimentManager already uses the EvalCallback, so am I replacing the EvalCallback with something like RawStatsEvalCallback?

[araffin]

@araffin also suggested I do the same for EvalCallback.

ah no, there was some misunderstanding on that.

[araffin]

@vwxyzjn I had to do a PR to your fork as I couldn't push to it :/

[araffin]

Could you check what I did?
if you add test and update changelog, we are good to go ;)

[vwxyzjn]

Added a test case and updated the changelog :)

[araffin]

LGTM =)

[vwxyzjn]

Awesome. Fell free to merg

[vwxyzjn]

Awesome. Feel free to merge