[Refactor] Simplify LSTM/GRUModule recurrent-mode shape normalization

[vmoens]

Stack from ghstack (oldest at bottom):

• [Doc] Migrate shifted=True callers to legacy/compact + docstring polish #3776
• [Example] Expose compact GAE cat dimension #3775
• [Example] Add Isaac RNN PPO rollout mode flags #3769
• [Example] Isaac Lab RNN PPO with compact memory + knowledge-base notes #3763
• [BugFix] Fix GAE compact path bias on recurrent value nets at internal truncations #3771
• -> [Refactor] Simplify LSTM/GRUModule recurrent-mode shape normalization #3768
• [Refactor] Keep [B, T] dim in value estimators #3767
• [Feature] Collector.fake_tensordict() / MultiCollector.fake_tensordict() #3764
• [Feature] MultiCollector: track policy version per fresh continue command #3759
• [BugFix] GAE shifted=True: tolerate missing next obs, non-canonical strides, docs #3757
• [Feature] timeit.mark_start/mark_end for non-context-manager timing #3762
• [BugFix] PolicyVersion: int64 dtype + preserve tensordict device #3755
• [Performance] LSTM/GRU scan: canonical strides + cuDNN flat-storage clones + thread-local recurrent mode #3754
• [BugFix] Recurrent policy auto-register with policy_factory #3753

[pytorch-bot]

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