adding a custom caster for c10::SymInt (#82692)
### Description
Adding a custom caster for `c10::SymInt`. This simplifies handling of c10::SymInt on C++/Pytorch boundary. Namely, removing if statements to handle the union nature (e.g. SymIntNode, int) of c10::SymInt.
### Issue
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### Testing
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Pull Request resolved: https://github.com/pytorch/pytorch/pull/82692
Approved by: https://github.com/ezyang
