Add tests for removing unused nodes in subgraphs

[justinchuby]

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[codecov]

❌ 8 Tests Failed:

Tests completed	Failed	Passed	Skipped
5871	8	5863	3238

View the top 3 failed test(s) by shortest run time

onnxscript.ir.passes.common.unused_removal_test.RemoveUnusedTest_0::test_remove_unused_optional_outputs_in_subgraph

Stack Traces | 0.001s run time

.../passes/common/unused_removal_test.py:366: in test_remove_unused_optional_outputs_in_subgraph
    model = onnx.parser.parse_model(
..../test/lib/python3.10.../site-packages/onnx/parser.py:25: in parse_model
    raise ParseError(msg)
E   onnx.parser.ParseError: b'[ParseError at position (line: 6 column: 51)]\nError context:                 z = If (x) <then_branch=then_graph, else_branch=else_graph>\nExpected character = not found.'

onnxscript.ir.passes.common.unused_removal_test.RemoveUnusedTest_0::test_unused_inputs_in_subgraph_are_not_removed

Stack Traces | 0.001s run time

.../passes/common/unused_removal_test.py:332: in test_unused_inputs_in_subgraph_are_not_removed
    model = onnx.parser.parse_model(
..../test/lib/python3.10.../site-packages/onnx/parser.py:25: in parse_model
    raise ParseError(msg)
E   onnx.parser.ParseError: b'[ParseError at position (line: 6 column: 51)]\nError context:                 z = If (x) <then_branch=then_graph, else_branch=else_graph>\nExpected character = not found.'

onnxscript.ir.passes.common.unused_removal_test.RemoveUnusedTest_1::test_remove_unused_initializers_in_subgraph

Stack Traces | 0.001s run time

.../passes/common/unused_removal_test.py:289: in test_remove_unused_initializers_in_subgraph
    model = onnx.parser.parse_model(
..../test/lib/python3.10.../site-packages/onnx/parser.py:25: in parse_model
    raise ParseError(msg)
E   onnx.parser.ParseError: b'[ParseError at position (line: 6 column: 51)]\nError context:                 z = If (x) <then_branch=then_graph, else_branch=else_graph>\nExpected character = not found.'

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