Fix envpool bug with missing mask key

[arjun-kg]

Description

There is a bug when using latest gym (v0.25.1) with envpool. See #3021.

This arises since gym requires _X mask key to be present in infos to indicate if element i in infos[X] exists. But this is not a requirement in envpool. However as mentioned in the issue thread, for truncated, when terminated=True, we don't care about the value of truncated, therefore we can practically treat the key not existing to be the same as the key existing and being set to False. The compatibility function is changed to reflect this. truncated is set to False when this occurs, even in list infos (old vector infos api) for consistency. This leads to slight loss of information, but again practically should not affect anything. And this is only through compatibility functions, when using new API exclusively, this problem does not occur, terminated=True, truncated=True can occur together as these are independent events.

Fixes #3021

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• I have run the pre-commit checks with pre-commit run --all-files (see CONTRIBUTING.md instructions to set it up)
• I have commented my code, particularly in hard-to-understand areas
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[pseudo-rnd-thoughts]

LGTM