Remove deprecated `TiedEmbeddingTransformerDecoder`

Context

What is the purpose of this PR? Is it to

[ ] add a new feature
[ ] fix a bug
[ ] update tests and/or documentation
[x] other (please add here)

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

Test plan

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UX

If your function changed a public API, please add a dummy example of what the user experience will look like when calling it. Here is a docstring example and a tutorial example

[x] I did not change any public API
[ ] I have added an example to docs or docstrings

:link: Helpful Links

:test_tube: See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/torchtune/2047

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As of commit 3fd14e1e5d711c6b9988e24046849dd25f78770c with merge base e9fd56a812cf0ba151fa164a45eb04056d099726 (): :green_heart: Looks good so far! There are no failures yet. :green_heart:

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pytorch / torchtune