Fix evaluation script for recurrent policies

[araffin]

Description

required for https://github.com/Stable-Baselines-Team/stable-baselines3-contrib/tree/feat/ppo-lstm

Motivation and Context

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

related to #18

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)
• I have checked that the documentation builds using make doc (required)

Note: You can run most of the checks using make commit-checks.

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

[Miffyli]

Couple of comments but otherwise LGTM :)