refactor output tracing for codegen

[omkar-334]

This PR refactors the codegen model as per #43979

cc @molbap

2 tests are being skipped because this model is not being loaded -

WARNING  tests.test_pipeline_mixin:test_pipeline_mixin.py:386 CodeGenModelTest::test_pipeline_feature_extraction_float32 is skipped: Could not find or load the model from `hf-internal-testing/tiny-random-CodeGenModel` with `<class 'transformers.models.codegen.modeling_codegen.CodeGenModel'>`.

[omkar-334]

@molbap this is being overwritten by make fix-repo because this was Copied from transformers.models.gptj.modeling_gptj.GPTJBlock with GPTJ->CodeGen. Looks like we need to merge that first to fix this. This is the reason for the failing test.

[github-actions]

[For maintainers] Suggested jobs to run (before merge)

run-slow: codegen