Fix quantized max_pool2d output observer for tuple-returning ops (#19259)

[abeakkas]

Summary:

prepare_pt2e skips the output observer for max_pool2d_with_indices because its output is a tuple, not a single tensor. This caused the quantized_max_pool2d fusion to silently fail.

Fix: annotate getitem[0] (single tensor) instead of the tuple-returning op. Also bail out if indices are consumed or the graph structure is unexpected.

Reviewed By: mcremon-meta

Differential Revision: D103436172

[pytorch-bot]

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❌ 3 New Failures, 1 Cancelled Job, 3 Unrelated Failures

As of commit 1317bf1 with merge base 8464b47 ():

NEW FAILURES - The following jobs have failed:

• Build Presets / windows (pybind) / build (gh)
• pull / test-qnn-testsuite-linux / test-backend-linux (qnn, operators) / linux-job (gh)
  RuntimeError: Command docker exec -t f79208fc4279a0e66cd20aa304ea845df3036a8a3dba9f23c858027397d8811c /exec failed with exit code 92
• pull / unittest-nxp-neutron / linux-job (gh)
  RuntimeError: Command docker exec -t bf03e91bb26fb2ea4fae56344ed70b6f534b371cefc8ec1d9ffb84ce1b3e2454 /exec failed with exit code 1

CANCELLED JOB - The following job was cancelled. Please retry:

• Build Presets / windows (windows) / build (gh)
  The runner has received a shutdown signal. This can happen when the runner service is stopped, or a manually started runner is canceled.

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• pull / unittest-editable / windows / windows-job (gh) (detected as infra flaky with no log or failing log classifier)

BROKEN TRUNK - The following jobs failed but was present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / unittest / macos / macos-job (gh) (trunk failure)
  ##[error]The operation was canceled.
• pull / unittest-editable / macos / macos-job (gh) (trunk failure)
  ##[error]The operation was canceled.

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[meta-codesync]

@abeakkas has exported this pull request. If you are a Meta employee, you can view the originating Diff in D103436172.

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