Document prefix_tokens arg

Author: sjmonson

docs/datasets.md

@@ -69,6 +69,7 @@ guidellm benchmark \
 - `output_tokens_stdev`: Standard deviation for output tokens. If not supplied and min/max are not specified, no deviation is applied. If not supplied and min/max are specified, a uniform distribution is used.
 - `output_tokens_min`: Minimum number of tokens in outputs. If unset and `output_tokens_stdev` is set, the minimum is 1.
 - `output_tokens_max`: Maximum number of tokens in outputs. If unset and `output_tokens_stdev` is set, the maximum is 5 times the standard deviation.
+- `prefix_tokens`: Number of tokens to share as a prefix across all prompts. Is additive to the prompt tokens distribution so each request is `prefix_tokens + prompt_tokens_sample()`. If unset, defaults to 0.
 - `samples`: Number of samples to generate (default: 1000). More samples will increase the time taken to generate the dataset before benchmarking, but will also decrease the likelihood of caching requests.
 - `source`: Source text for generation (default: `data:prideandprejudice.txt.gz`). This can be any text file, URL containing a text file, or a compressed text file. The text is used to sample from at a word and punctuation granularity and then combined into a single string of the desired lengths.