[Feature] SignTransform

[albertbou92]

Description

This PR adds a new Transform called SignTransform. As explained in the docs,

    This transform reads the tensors in ``in_keys`` and ``in_keys_inv``, computes the
    signs of their elements and writes the resulting sign tensors to ``out_keys`` and
    ``out_keys_inv`` respectively.

Motivation and Context

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If it fixes an open issue, please link to the issue here.
You can use the syntax close #15213 if this solves the issue #15213

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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)

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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.

[pytorch-bot]

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✅ You can merge normally! (27 Unrelated Failures)

As of commit d7b7413 with merge base f4878c9 ():

FLAKY - The following jobs failed but were likely due to flakiness present on trunk:

• Continuous Benchmark (PR) / CPU Pytest benchmark (gh)
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• Continuous Benchmark (PR) / GPU Pytest benchmark (gh)
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• Examples Tests on Linux / tests (3.9, 12.1) / linux-job (gh)
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• Generate documentation / build-docs (3.9, 12.1) / linux-job (gh)
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• Habitat Tests on Linux / tests (3.9, 11.6) / linux-job (gh)
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• Libs Tests on Linux / unittests-brax (3.9, 12.1) / linux-job (gh)
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• Unit-tests on Linux / tests-gpu (3.8, 12.1) / linux-job (gh)
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• Unit-tests on Linux / tests-stable-gpu (3.8, 11.8) / linux-job (gh)
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[vmoens]

LGTM, just a very minor edit