Remove deprecated TiedEmbeddingTransformerDecoder

[EmilyIsCoding]

Context

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• add a new feature
• fix a bug
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Please link to any issues this PR addresses.
Closes #2041

Changelog

What are the changes made in this PR?

• Removed deprecated TiedEmbeddingTransformerDecoder and unused imports in torchtune/modules/transformer.py
• Removed unused TiedEmbeddingTransformerDecoder imports in torchtune/modules/__init__.py

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UX

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[pytorch-bot]

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[RdoubleA]

deleting code is always beautiful. will merge after CI is green