[Feature] Add option for auto-resetting envs in GAE

[lin-erica]

Description

Previously, when calculating the GAE, torchrl assumes that there is a valid s_(t+1) return from the environment. However, for auto-resetting environments, that is not true at the end of the episode, as it just jumps to the first state of the next episode. This PR adds a feature to the GAE class, allowing users to specify if it's an auto-resetting env, in which case it will use the s_t as an approximation for bootstrapping.

Motivation and Context

Without this change, we were getting much slower convergence rates during training for IsaacLab (an autoresetting env).

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[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/rl/2851

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[lin-erica]

@vmoens Perhaps I missed it, but I didn't see any tests that were testing the GAE class. If there is, I'm happy to add a relevant test as well.