Rename the observations variable in the evaluation util to avoid shadowing

[npit]

This is identical to DLR-RM/stable-baselines3#1288

Note that I can't complete all tests as I can't test
Tests ok.

This is a simple renaming. It enables a callback in evaluate_policy to have access to the observation vector that is fed to the environment step function, which is currently shadowed by the output observation.

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[npit]

Update: all tests pass. Updated the top post.