Preserve nested control-flow subgraphs during pt2e convert

[atharvjairath]

Fixes #4455.

GraphModule.delete_all_unused_submodules() only inspects the immediate graph, so convert() could delete submodules that are referenced only from nested control-flow subgraphs. In nested while_loop cases, this left dangling references and caused DuplicateDQPass graph linting to fail.

This adds a recursion-aware cleanup wrapper that preserves submodules referenced from nested GraphModule children, and adds a regression test covering nested while_loop conversion.

Test plan:

• python -m ruff check torchao/quantization/pt2e/convert.py test/quantization/pt2e/test_quantize_pt2e.py
• python -m pytest test/quantization/pt2e/test_quantize_pt2e.py::TestQuantizePT2E::test_nested_while_loop_convert_preserves_subgraphs -q
• python -m pytest test/quantization/pt2e/test_duplicate_dq.py -q
• git diff --check

[pytorch-bot]

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