Add `agent.pt` and tag the environment

This commit adds a model that is compatible with the 🥇 [Open RL Leaderboard](https://huggingface.co/spaces/open-rl-leaderboard/leaderboard) 🥇.

README.md

@@ -5,6 +5,7 @@ tags:
 - deep-reinforcement-learning
 - reinforcement-learning
 - stable-baselines3
+- HalfCheetah-v4
 model-index:
 - name: TRPO
   results:

agent.pt

@@ -1,3 +1,3 @@
 version https://git-lfs.github.com/spec/v1
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-size 27558
+oid sha256:7afae92406308e1494176b7c4607b7a0d741842e0d2adbd340104299cd0feffb
+size 28810