[Refactor] Refactor FP8 & INT8 Quant Folder inside 8bit

[yewentao256]

Purpose

Refactor FP8 & INT8 Quant Folder inside 8bit for better code maintainability

Test

lm_eval   --model vllm   --model_args "pretrained=Qwen/Qwen3-30B-A3B-FP8,max_model_len=32768,enforce_eager=True"   --trust_remote_code   --tasks gsm8k   --num_fewshot 5   --batch_size auto
vllm (pretrained=Qwen/Qwen3-30B-A3B-FP8,max_model_len=32768,enforce_eager=True,trust_remote_code=True), gen_kwargs: (None), limit: None, num_fewshot: 5, batch_size: auto
|Tasks|Version|     Filter     |n-shot|  Metric   |   |Value |   |Stderr|
|-----|------:|----------------|-----:|-----------|---|-----:|---|-----:|
|gsm8k|      3|flexible-extract|     5|exact_match|↑  |0.8279|±  |0.0104|
|     |       |strict-match    |     5|exact_match|↑  |0.8961|±  |0.0084|
# main
|Tasks|Version|     Filter     |n-shot|  Metric   |   |Value |   |Stderr|
|-----|------:|----------------|-----:|-----------|---|-----:|---|-----:|
|gsm8k|      3|flexible-extract|     5|exact_match|↑  |0.8264|±  |0.0104|
|     |       |strict-match    |     5|exact_match|↑  |0.8954|±  |0.0084|

[github-actions]

👋 Hi! Thank you for contributing to the vLLM project.

💬 Join our developer Slack at https://slack.vllm.ai to discuss your PR in #pr-reviews, coordinate on features in #feat- channels, or join special interest groups in #sig- channels.

Just a reminder: PRs would not trigger full CI run by default. Instead, it would only run fastcheck CI which starts running only a small and essential subset of CI tests to quickly catch errors. You can run other CI tests on top of those by going to your fastcheck build on Buildkite UI (linked in the PR checks section) and unblock them. If you do not have permission to unblock, ping simon-mo or khluu to add you in our Buildkite org.

Once the PR is approved and ready to go, your PR reviewer(s) can run CI to test the changes comprehensively before merging.

To run CI, PR reviewers can either: Add ready label to the PR or enable auto-merge.

🚀

[gemini-code-assist]

Code Review

This pull request refactors the FP8 and INT8 quantization code into a new 8bit directory, which improves code organization and maintainability. The changes primarily involve moving files, updating include paths, and cleaning up some comments. The logic appears to be preserved correctly. I have one suggestion regarding a removed comment that provided important context for preventing potential integer overflow issues.

[yewentao256]

@mgoin CC