Refine the LLM manual (focus on the debugging and profiling part)

[pytorchbot]

Summary:

• Remove imports that have been imported in previous part of the doc
• Other minor changes to keep consistency across the doc
• Some auto-formatting (remove extra spaces)
• Link a screenshot because the original table is illegible:
  {F1482781056}

Differential Revision: D55938344

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/2971

• 📄 Preview Python docs built from this PR

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❌ 2 New Failures

As of commit 1456abd with merge base d3326a2 ():

NEW FAILURES - The following jobs have failed:

• pull / test-models-linux (cmake, llava_encoder, portable, linux.4xlarge, 90) / linux-job (gh)
  RuntimeError: Command docker exec -t 16712d0b82877b09ee4d57d042c449edf553ccd76a5cd2c957c8130ac19f9da3 /exec failed with exit code 1
• pull / test-selective-build-linux (buck2) / linux-job (gh)
  ##[error]The operation was canceled.

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