feat: add --disable-prompt-logprobs argument

[tjohnson31415]

Description

Adds a new --disable-prompt-logprobs boolean argument that defaults to False. When set to True, prompt_logprobs will always be set to None in the SamplingParams, regardless of the incoming request parameters, disabling all usage of prompt logprobs.

This flag is being added because computation of prompt logprobs can add latency to request processing and there are a couple of bugs/crashes that occur when using prompt logprobs.

How Has This Been Tested?

Manually tested to verify that the flag can be set via env var and that it has the desired effect to not return prompt logprobs (but still returns logprobs on generated tokens).

Merge criteria:

• The commits are squashed in a cohesive manner and have meaningful messages.
• Testing instructions have been added in the PR body (for PRs involving changes that are not immediately obvious).
• The developer has manually tested the changes and verified that the changes work

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 62.08%. Comparing base (1642b30) to head (4b4bd4e).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #95      +/-   ##
==========================================
+ Coverage   62.02%   62.08%   +0.06%     
==========================================
  Files          21       21              
  Lines        1214     1216       +2     
  Branches      213      213              
==========================================
+ Hits          753      755       +2     
  Misses        386      386              
  Partials       75       75              

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