Get decompositions only for CIA ops

[HolyWu]

Description

torch.export.default_decompositions() will get the same opset of decompositions as torch._decomp.core_aten_decompositions(), which is what you get when enable_experimental_decompositions=True. That is unwanted in enable_experimental_decompositions=False path. We only need decompositions for CIA ops here.

Checklist:

• My code follows the style guidelines of this project (You can use the linters)
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas and hacks
• I have made corresponding changes to the documentation
• I have added tests to verify my fix or my feature
• New and existing unit tests pass locally with my changes
• I have added the relevant labels to my PR in so that relevant reviewers are notified

[peri044]

LGTM. Based on my understanding, the three tables with this change are (which used to be the case before Pytorch upstream changes)

1. Fixed set of torch decompositions that we maintain (since upstream can keep changing)
2. All CIA op decompositions
3. Our custom decompositions