Fix QNN online-prepare graph handle retrieval

[sylvesterkaczmarek]

Fixes #21804.

QnnContext::Configure() registers DLC graphs into the QNN context for ONLINE_PREPARE before QnnGraph::Configure() runs. The graph configuration path then skipped both graph creation and graph retrieval, leaving graph_handle null and storing it in handle_.

This change retrieves already-registered DLC graphs with qnn_graph_retrieve, matching the deserialization path, applies AfterRetrieveGraph, and defensively rejects a null graph handle before bookkeeping.

Validation: checked the initialization sequence through QnnContext::Configure() and QnnManager::InitContext(), and kept the serialize/multi-graph creation paths unchanged.

[pytorch-bot]

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⚠️ 12 Awaiting Approval, 1 Pending

As of commit 487b3ea with merge base ed65b12 ():

AWAITING APPROVAL - The following workflows need approval before CI can run:

• Build documentation (gh)
• Build Presets (gh)
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• Test ARM Backend (gh)
• Test CoreML Backend (gh)
• Test QNN Backend (gh)
• Test QNN Windows MSVC build (gh)
• Test Vulkan Backend (gh)
• Test WebGPU Backend (gh)
• Test XNNPACK Backend (gh)

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[sylvesterkaczmarek]

@pytorchbot label "release notes: qualcomm"

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