[CI Bugfix] Fix test_sequence missing create_dummy_prompt

[mgoin]

Purpose

Add back missing create_dummy_prompt function to test_sequence.py removed in #25082 to fix Engine Test https://buildkite.com/vllm/ci/builds/31153/steps/canvas?sid=01995896-c6f7-469d-9582-e6a8570819ae

Test Plan

Test Result

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• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
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[gemini-code-assist]

Code Review

This pull request re-introduces the create_dummy_prompt function to test_sequence.py to fix a CI failure. My review identifies a critical issue in the implementation where block_size can become negative, leading to potential crashes or incorrect behavior. I've provided a code suggestion to fix this by ensuring block_size is always a positive integer and correctly derived from the prompt's length.

[gemini-code-assist]

The logic for determining block_size is flawed and can lead to a negative value, which is invalid for a Sequence. Specifically:

1. If prompt_tokens is provided, prompt_length defaults to -1, causing block_size to be set to -1.
2. A negative block_size can cause incorrect calculations for the number of blocks, or even crashes (e.g., division by zero if it were 0).
3. The comment on line 33 is inconsistent with the code, as it refers to block_size for generating tokens while the code uses prompt_length.

I suggest refactoring this block to correctly determine prompt_tokens and block_size, ensuring block_size is always a positive integer.

    if prompt_tokens is None:
        prompt_tokens = list(range(max(0, prompt_length)))

    if block_size is None:
        if prompt_embeds is not None:
            block_size = len(prompt_embeds)
        else:
            block_size = len(prompt_tokens)

    if block_size <= 0:
        block_size = 1