Fix Inspector crash on multi-method ETRecords

[u7k4rs6]

Summary

The Inspector can't be built from an ETRecord of a multi-method program. Since #17185, ETRecord stores edge_dialect_program as a dict of method name -> program when there's more than one method, but gen_graphs_from_etrecord and _resolve_reference_graph still do .graph_module on it, so Inspector(etdump_path=..., etrecord=...) fails right away with AttributeError: 'dict' object has no attribute 'graph_module'.

This adds a small helper, get_forward_edge_dialect_program, that returns the program as-is when it's a single one, or the "forward" entry when it's a dict, and uses it in those two places. The Inspector already only correlates the forward method, so that's the graph it should use.

Programs with no method named "forward" still fail earlier in ETRecord handling (KeyError: 'forward'). That needs a bigger decision about which method to inspect, so I left it out of this one.

Test plan

Added test_inspector_with_multi_method_etrecord to devtools/inspector/tests/inspector_test.py. It exports a two-method program (forward + times_two) with generate_etrecord=True, builds an Inspector from the saved ETRecord, and checks the reference graph is the forward graph. It fails on main with the AttributeError above and passes with the fix, and the rest of devtools/inspector/tests passes. ufmt and flake8 are clean with the versions from requirements-lintrunner.txt.

cc @nil-is-all

[pytorch-bot]

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[u7k4rs6]

@pytorchbot label "release notes: devtools"

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