Document Quantizer API and precision options for MTK backend

[pytorchbot]

Added Quantizer API section with precision configuration example.

[pytorch-bot]

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🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/15203

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❌ 4 New Failures

As of commit fae3ac6 with merge base e0dda90 ():

NEW FAILURES - The following jobs have failed:

• pull / test-llava-runner-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 54bb864458887690a3b03bc342d8aaa3ca6fd9cfe3717badb65488dde53abf03 /exec failed with exit code 1
• pull / unittest / macos / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1
• pull / unittest-buck / macos / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1
• pull / unittest-editable / macos / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1

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