Reenable and improve preprocess dataset

[jaredoconnell]

Summary

This PR re-enables, tests, and documents the preprocess dataset command.
Also changes the format that prompt and output sizes are specified, and makes the code aware of prefixes.

Details

• Uses the post-refactor code to re-enable the command.
• Switches over to the same format used by benchmark run's synthetic data for the data config to enable more features and make the command more cohesive with the rest of GuideLLM.
• Adds options for prefixes. I added an option to include prefixes in the count, since prefixes are included in input and output tokens, and affect performance.

Test Plan

• Run with a known dataset, or create one as a simple CSV.
• New tests are added that should cover everything except huggingface uploads. They are all at least in part generated by AI, but I went through each one iteratively to ensure they do what they need to do.

---

• "I certify that all code in this PR is my own, except as noted below."

Use of AI

• Includes AI-assisted code completion
• Includes code generated by an AI application
• Includes AI-generated tests (NOTE: AI written tests should have a docstring that includes ## WRITTEN BY AI ##)

[sjmonson]

I think the reuse of SyntheticTextDatasetConfig is not a great idea. prefix_buckets are defined differently here. For synthetic data a prefix bucket of prefix_tokens=10,prefix_count=1 means you get one identical prefix for the entire dataset. As implemented here prefix_tokens=10,prefix_count=1 will only ensure that every row has a prefix of length 10. It does not guarantee any shared prefix between rows.

Rather then reuse SyntheticTextDatasetConfig I think the best option is to create a new config format that is similar only where it makes sense. For example:

prompt_tokens:
prompt_tokens_stdenv:
prompt_tokens_min:
prompt_tokens_max:
output_tokens:
output_tokens_stdenv:
output_tokens_min:
output_tokens_max:
prefix_tokens:
prefix_tokens_stdenv:
prefix_tokens_min:
prefix_tokens_max:

Treat prefix the same as prompt and output.

[sjmonson]

For better organization I think these handle_*_strategy methods should be defined on a static class.

[jaredoconnell]

Done.

[jaredoconnell]

I moved it to its on class in a way that retains a single source of truth so that we can use the same documentation. I just simplified it to only have the option of trimming prefixes. I decided that it wouldn't make sense to use a randomized size sampling because that's not how samples work in real scenarios typically.

[sjmonson]

Need to double-check that benchmark run is unaffected but LGTM.