[Feature] Add EnsembleModule

[smorad]

Description

This enables the use of functional ensembles in tensordict. I'm having trouble building the docs, but I'm not doing anything crazy with the docs so it should "just work".

Motivation and Context

I moved pytorch/rl#1359 to the tensordict repo as it could be useful outside of RL scenarios. This makes progress on pytorch/rl#1344.

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Types of changes

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• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds core functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (update in the documentation)
• Example (update in the folder of examples)

Checklist

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• 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.

[vmoens]

Neat! Thanks for this
One minor thing regarding headers and we're good to merge (if tests pass)

[vmoens]

Also: can you fix the lint with pre-commit?