Gym Env Checker

[araffin]

Description

Many people have errors because of a custom gym environment, this should address most issues
related to a wrong implementation of the Gym interface.

Motivation and Context

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

closes #601
related to #595

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 ensured pytest and pytype both pass.

[AdamGleave]

Great feature, glad we have this, will catch a lot of common problems.

High-level feedback is I think this would be clearer if check_env (and tests) were split up into smaller helper methods. Right now if I wanted to add a check I'm not sure it'd take me a while to figure out the right place.

Otherwise looks good, made a few minor comments.

[araffin]

High-level feedback is I think this would be clearer if check_env (and tests) were split up into smaller helper methods.

Good point, I will do that, thanks for taking the time to review =)

will catch a lot of common problems.

In fact, we should have created that way earlier ( we have more that 50 issues related to custom environments...)

[AdamGleave]

Thanks for the changes, it's much clearer now that things have been split into smaller methods.

Made some minor suggestions (mostly explicitly annotating None return type). One thing I'm confused by: why do we need to check method_name == 'reset'?

Otherwise LGTM.

[araffin]

I reformated the files using pycharm + addressed the comments ;) (+ fix the type in check_image)

[AdamGleave]

Thanks, LGTM!