[Doc] Document Composite.keys() nested walk order

[YeonwooSung]

Description

Documents the nested walk order of :meth:~torchrl.data.Composite.keys`` as a public contract.

keys(include_nested=True, leaves_only=False) follows the same depth-first walk as :meth:tensordict.TensorDict.keys: each nested `:class:`~torchrl.data.Composite is yielded after its children (child before parent), and siblings keep insertion order. The previous docstring listed ["next", ("next", "obs")] as the set of keys, which read as parent-before-child; the runtime order is [("next", "obs"), "next"]. That order is now stated explicitly, with a copy-paste example. :class:~torchrl.data.StackedComposite`` keys() gets the same note.

The walk is not changed. Empty nested specs can be dropped in one pass over list(spec.keys(True, False)) because children are visited first.

Motivation and Context

close #2482

The issue asked whether CompositeSpec.keys(True, False) is deepest-first so empty nested specs can be deleted in a single scan. It is, matching TensorDict. This PR records that guarantee instead of adding prune_empty() or reversing the walk.

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Types of changes

• Documentation (update in the documentation)

Checklist

• I have read the CONTRIBUTION guide (required)
• My change requires a change to the documentation.
• I have updated the tests accordingly (required for a bug fix or a new feature).
• I have updated the documentation accordingly.

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/rl/4477

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