Updating generation to use custom_generate_next_token for initial token generation

[SalmanMohammadi]

Context

What is the purpose of this PR? Is it to

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

I'n using generate with a custom custom_generate_next_token defined for a model which outputs both the output of TransformerDecoder and an additional value head. generate fails when making the first call to generate_next_token as it encounters an unexpected return signature for model(...).

Let me know if there's a better way for me to do this.

Changelog

Updated the initial call to generate_next_token to use custom_generate_next_token.

Test plan

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• [ ] add unit tests for any new functionality
• [ ] update docstrings for any new or updated methods or classes
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• [ ] run recipe tests via pytest tests -m integration_test
• [ ] manually run any new or modified recipes with sufficient proof of correctness
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[pytorch-bot[bot]]

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As of commit a605d9490ea1dccaa708c5828fe6fca44533c08e with merge base dc2b9911cf5a793a8b4f9cd4df321c5271745b4a (): :green_heart: Looks good so far! There are no failures yet. :green_heart:

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[SalmanMohammadi]

Thanks so much for the review. I think I kind of get the gist of the original design choice - I'll ping any qs on discord : )