[Feature] Add checkpoint rotation and retention

[theap06]

##Summary:

• This PR adds automatic checkpoint retention and rotation to TorchRL.
• Previously, Checkpoint.save(path) could either overwrite the same path or require users to create and delete versioned checkpoints themselves. The new CheckpointRotation class manages that lifecycle.

[pytorch-bot]

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🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/rl/4098

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⚠️ 15 Awaiting Approval

As of commit 65c84b6 with merge base ae421b9 ():

AWAITING APPROVAL - The following workflows need approval before CI can run:

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[theap06]

@torchrlbot reviewer @vmoens

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