[ONNX] Refactor ONNX fixup for Loop and If (#40943)
Summary:
* move both under new file `fixup_onnx_controlflow`
* move the fixup to where the ONNX loop/if node is created, as oppose to running the fixup as postpass. This will help with enable onnx shape inference later.
* move `fuseSequenceSplitConcat` to `Peephole`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/40943
Reviewed By: mrshenli
Differential Revision: D22709999
Pulled By: bzinodev
fbshipit-source-id: 51d316991d25dc4bb4047a6bb46ad1e2401d3d2d
