Speedup handling of terminal observations

[araffin]

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Description

python -m timeit -s "import numpy as np; a=np.ones(4096)" "a.nonzero()[0]"

10000 loops, best of 5: 12.4 usec per loop

vs

python -m timeit -s "import numpy as np; a=np.ones(4096)" "for idx in enumerate(a):pass"

1000 loops, best of 5: 150 usec per loop

but at the end, it's hard to see any speedup so far (will try to test more with massive parallel simulation). However, it makes the code slightly cleaner.

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 opened an associated PR on the SB3-Contrib repository (if necessary)
• [ ] I have opened an associated PR on the RL-Zoo3 repository (if necessary)
• [ ] 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)
• [ ] I have checked that the documentation builds using make doc (required)

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