[Feature] Entropy calculation support

[Deleter-D]

Motivation

💡 If this PR is a Cherry Pick, the PR title needs to follow the format by adding the [Cherry-Pick] label at the very beginning and appending the original PR ID at the end. For example, [Cherry-Pick][CI] Add check trigger and logic(#5191)

💡 如若此PR是Cherry Pick，PR标题需遵循格式，在最开始加上[Cherry-Pick]标签，以及最后面加上原PR ID，例如[Cherry-Pick][CI] Add check trigger and logic(#5191)

In reinforcement learning (RL) scenarios, entropy is an important diagnostic metric for model training. So we support entropy calculation in FD for debugging and analysis.

Modifications

This PR adds entropy calculation logic and introduces a service startup parameter to control whether entropy is computed.

Usage or Command

When starting the service, use the --enable-entropy flag to enable entropy calculation.
Once enabled, entropy-related information will be logged to data_processor.log, for example:

INFO     2025-12-22 20:21:54,906 178857 entropy_utils.py[line:96] req_id: chatcmpl-b141a269-d832-470c-b6b2-45a182a7c7a2_0, entropy: 0.8665663838614539

Accuracy Tests

Checklist

• Add at least a tag in the PR title.
  • Tag list: [[FDConfig],[APIServer],[Engine], [Scheduler], [PD Disaggregation], [Executor], [Graph Optimization], [Speculative Decoding], [RL], [Models], [Quantization], [Loader], [OP], [KVCache], [DataProcessor], [BugFix], [Docs], [CI], [Optimization], [Feature], [Benchmark], [Others], [XPU], [HPU], [GCU], [DCU], [Iluvatar], [Metax]]
  • You can add new tags based on the PR content, but the semantics must be clear.
• Format your code, run pre-commit before commit.
• Add unit tests. Please write the reason in this PR if no unit tests.
• Provide accuracy results.
• If the current PR is submitting to the release branch, make sure the PR has been submitted to the develop branch, then cherry-pick it to the release branch with the [Cherry-Pick] PR tag.

[paddle-bot]

Thanks for your contribution!

[codecov-commenter]

Codecov Report

❌ Patch coverage is 94.28571% with 4 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (develop@c1aa66d). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
fastdeploy/model_executor/pre_and_post_process.py	33.33%	2 Missing and 2 partials ⚠️

Additional details and impacted files

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  Coverage           ?   65.13%           
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  Hits               ?    27263           
  Misses             ?    12556           
  Partials           ?     2034           

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