[pt2] Add some helper function for SymIntVector (#101056)
Summary: To SymInt-ify some fbgemm kernels https://github.com/pytorch/FBGEMM/blob/f4c83b4fb362683c19afd71c5083ea6615e1d601/fbgemm_gpu/src/jagged_tensor_ops_autograd.cpp#L109, we need to provide a toSymIntVector helper function in pytorch
Test Plan: Run fbgemm op + torch.compile to make sure the symbolic shape is produced.
Differential Revision: D45724091
Pull Request resolved: https://github.com/pytorch/pytorch/pull/101056
Approved by: https://github.com/suo
