Enable ONNX backend tests on [U]INT4 types in Cast

[mcollinswisc]

Description

Support for [U]INT4 types in Cast op was added in:
51b678f

The comment on the skipped tests say they need ONNX 1.19. The ONNX submodule now points to 1.20.1:
de398c4

And if I run each of these they seem to work now:

$ ./build/Linux/RelWithDebInfo/onnx_test_runner cmake/external/onnx/onnx/backend/test/data/node/test_cast_*INT4*
Load Test Case: cast_UINT4_to_UINT8 in cmake/external/onnx/onnx/backend/test/data/node/test_cast_UINT4_to_UINT8
Load Test Case: cast_UINT4_to_FLOAT16 in cmake/external/onnx/onnx/backend/test/data/node/test_cast_UINT4_to_FLOAT16
Load Test Case: cast_UINT4_to_FLOAT in cmake/external/onnx/onnx/backend/test/data/node/test_cast_UINT4_to_FLOAT
Load Test Case: cast_INT4_to_INT8 in cmake/external/onnx/onnx/backend/test/data/node/test_cast_INT4_to_INT8
Load Test Case: cast_INT4_to_FLOAT16 in cmake/external/onnx/onnx/backend/test/data/node/test_cast_INT4_to_FLOAT16
Load Test Case: cast_INT4_to_FLOAT in cmake/external/onnx/onnx/backend/test/data/node/test_cast_INT4_to_FLOAT
Load Test Case: cast_FLOAT_to_UINT4 in cmake/external/onnx/onnx/backend/test/data/node/test_cast_FLOAT_to_UINT4
Load Test Case: cast_FLOAT_to_INT4 in cmake/external/onnx/onnx/backend/test/data/node/test_cast_FLOAT_to_INT4
Load Test Case: cast_FLOAT16_to_UINT4 in cmake/external/onnx/onnx/backend/test/data/node/test_cast_FLOAT16_to_UINT4
Load Test Case: cast_FLOAT16_to_INT4 in cmake/external/onnx/onnx/backend/test/data/node/test_cast_FLOAT16_to_INT4
2026-02-11 22:48:10.236181534 [E:onnxruntime:Default, testcase_driver.cc:40 RunParallel] Running tests in parallel: at most 10 models at any time
2026-02-11 22:48:10.332387281 [E:onnxruntime:Default, testcase_driver.cc:62 RunModelsAsync] Running tests finished. Generating report
result: 
	Models: 10
	Total test cases: 10
		Succeeded: 10
		Not implemented: 0
		Failed: 0
	Stats by Operator type:
		Not implemented(0): 
		Failed:
Failed Test Cases:

Also removing an old TODO from
cd7112f

Motivation and Context

I was trying out INT4 support & ran into these comments. And expanding test coverage would be good.

[mcollinswisc]

@adrianlizarraga Does the support added since cd7112f meet the intention of this TODO?