[VLM] Avoid unnecessary tokenization

[DarkLight1337]

Instead of tokenizing the string mm_token * token_count, we can first tokenize mm_token by itself to mm_token_id before repeating it. This reduces the time to run the processing tests in test_common.py from 18 mins to 12 mins on my machine.

[github-actions]

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[ywang96]

LGTM!

On a side note - I think it can be sometimes confusing that we're using the same interface for replacing prompt and prompt token ids. I don't think we should create more subclasses/interfaces, but it would be great to have examples under the documentation of PromptReplacement to make it more developer-friendly.

[DarkLight1337]

LGTM!

On a side note - I think it can be sometimes confusing that we're using the same interface for replacing prompt and prompt token ids. I don't think we should create more subclasses/interfaces, but it would be great to have examples under the documentation of PromptReplacement to make it more developer-friendly.

Yeah I'm planning to add more docs soon.