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[Bugfix] Revert incorrect updates to num_computed_tokens #8950

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varun-sundar-rabindranath
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@varun-sundar-rabindranath varun-sundar-rabindranath commented Sep 30, 2024

Fix bug introduced in PR #8378 . num_computed_tokens only tracks the number of prefill tokens computed. The PR incorrectly updates num_computed_tokens for sampled tokens as well.

Thanks @LiuXiaoxuanPKU for catching this.


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LGTM. Do we have a unit test to catch this?

@comaniac comaniac added the ready ONLY add when PR is ready to merge/full CI is needed label Sep 30, 2024
@comaniac comaniac enabled auto-merge (squash) September 30, 2024 03:45
@LiuXiaoxuanPKU
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LiuXiaoxuanPKU commented Sep 30, 2024

I feel we need a clear definition of num_computed_tokens.
Although the comment says
"""Return the number of prefill tokens that are already computed."""

Before #8378, num_computed_tokens still tracks the number of computed tokens during the decoding phase.
After #8378, num_computed_tokens also tracks both the tokens during the prefill and decoding phase, but the problem is that it double counts the last prefill token.

More concretely, given prompt [1,2,3,4,5], after the prefill phase, we generate token 6,
before #8378, num_computed_tokens is 5.
after #8378, num_computed_tokens is 6.

Then we generate one token 7 during the decoding phase,
before #8378, num_computed_tokens is updated to 6.
after #8378, num_computed_tokens is updated to 7.

Maybe the definition of num_computed_tokens should be the number of tokens computed, both in the prefill phase and decoding phase? In that case, in the future, speculative decoding can also use this field.

@varun-sundar-rabindranath
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Closing in favor of #9038

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