[Bugfix] add truncate_prompt_tokens to work offline, directly from LLM class.

[yecohn]

Fixes #4507.

• added function _validate_prompt to make sure prompt is a right format and run some validations.
• truncate_prompt_tokens should take truncate the prompt to the left as seen in vllm/entrypoints/openai/serving_engine.py line 182.

[simon-mo]

@tdoublep can you help review this?

[tdoublep]

@simon-mo sure, will try to get to that later today

[tdoublep]

This function re-implements the same logic as this function. It's worth thinking about whether we should have it implemented in a single place and re-used accordingly. I think that would be preferable, unless there is some strong reason to have it duplicated.

[tdoublep]

I'm also studying #3512 to understand if that needs to be merged before this one.

[yecohn]

yes this is a good point. let me know what do you think is the best and I'll implement it

[tdoublep]

For now let's just make sure that the behaviour is equivalent (see the below discussion re: using truncation from the tokenizer). I think it would make more sense to think about refactoring it after #3512 is merged because after that point the completions API will be using the tokenizer from the engine anyway.

[tdoublep]

This isn't exactly the same behaviour as the equivalent code in the completion API. In that case, we use the built-in capability of the tokenizer to perform truncation in the case that the user provides a prompt, and only implement the truncation ourselves in the case when the user provides prompt ids. I think it would be better to do the same here to be consistent.

[yecohn]

yes indeed, the thing I remarked though is that using the tokenizer with kwargs does a right truncation and not a left one basically doing prompt_token_ids[:truncated_prompt_tokens]. I ll recheck to make sure I am not mistaken.

here the traceback I got:

`

Preparing working directory	2s
	# Creating "/workspace/build/buildkite/vllm/ci"
	$ cd /workspace/build/buildkite/vllm/ci
	$ git clone -v -- https://github.com/vllm-project/vllm.git .
	Cloning into '.'...
	POST git-upload-pack (175 bytes)
	POST git-upload-pack (gzip 2402 to 1225 bytes)
	remote: Enumerating objects: 16905, done.
	remote: Counting objects: 100% (3599/3599), done.
	remote: Compressing objects: 100% (711/711), done.
	remote: Total 16905 (delta 3181), reused 2987 (delta 2887), pack-reused 13306
	Receiving objects: 100% (16905/16905), 8.71 MiB | 20.47 MiB/s, done.
	Resolving deltas: 100% (12703/12703), done.
	$ git clean -ffxdq
	# Fetch and checkout pull request head from GitHub
	$ git fetch -- origin refs/pull/4598/head
	remote: Enumerating objects: 15, done.
	remote: Counting objects: 100% (15/15), done.
	remote: Compressing objects: 100% (10/10), done.
	remote: Total 15 (delta 7), reused 13 (delta 5), pack-reused 0
	Unpacking objects: 100% (15/15), 3.66 KiB | 749.00 KiB/s, done.
	From https://github.com/vllm-project/vllm
	* branch refs/pull/4598/head -> FETCH_HEAD
	# FETCH_HEAD is now 968d5e9651ca28b90498e282e3095f9654ceebe0
	$ git checkout -f 968d5e9
	Note: switching to '968d5e9651ca28b90498e282e3095f9654ceebe0'.
	You are in 'detached HEAD' state. You can look around, make experimental
	changes and commit them, and you can discard any commits you make in this
	state without impacting any branches by switching back to a branch.
	If you want to create a new branch to retain commits you create, you may
	do so (now or later) by using -c with the switch command. Example:
	git switch -c
	Or undo this operation with:
	git switch -
	Turn off this advice by setting config variable advice.detachedHead to false
	HEAD is now at 968d5e9 fix dependency injection
	# Cleaning again to catch any post-checkout changes
	$ git clean -ffxdq
	# Checking to see if Git data needs to be sent to Buildkite
	$ buildkite-agent meta-data exists buildkite:git:commit
	Running commands
	$ trap 'kill -- $$' INT TERM QUIT; cp -r ~/.ssh /workspace/.ssh && chmod -R 777 /workspace
	Some containers had unknown exit statuses. Perhaps they were in ImagePullBackOff.

`

[yecohn]

regarding the build I got an error buildkite/ci/pr/distributed-tests-multiple-groups — Failed (exit status -10) do you have any idea what could it be ? and how could I debug this ?

[tdoublep]

@yecohn Regarding the truncation side issue. You are right, that by default the tokenizers do truncation from the right. That's why we added this argument when calling get_tokenizer to set the truncation side to left.

Re: the build error I don't think it looks related to your changes currently.

[JackChuang]

Hi @tdoublep, @yecohn, and @simon-mo,
It looks like this feature has not been merged yet. According to my tests, truncate_prompt_tokens still does not work for offline inference.

I’d like to see this feature implemented. In my case, I need the truncation to occur in vllm, and I would also like to experiment with the generated KV caches.

Are there any plans to support this feature? Can I help in any way?
Or, alternatively, are there any other methods to perform prompt truncation while running offline inference in the vllm?

Thanks,