In this PR, we implement lazy dictionary for export decomp behaviour for following reasons:
1. Custom op loading can happen after import time, as a result, the decomp table might not be able to pick up the decomp. Therefore we try to delay materialization as late as possible.
I intentionally seperated out the core_aten_decomp to not have any custom CIA ops in this PR to mitigate the risk of getting reverted but in the future, core_aten_decomp under torch/_decomp will exist as an alias to official export table (torch.export.default_decompositions)
Differential Revision: [D64140807](https://our.internmc.facebook.com/intern/diff/D64140807)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/137650
Approved by: https://github.com/justinchuby, https://github.com/bdhirsh