[ONNX] Create decorator to handle symbolic context (#84776)
- Create decorator to handle old style custom symbolics that require context
- Deprecate `torch.onnx.SymbolicContext` in favor of `GraphContext`. Added deprecation message
- Remove README reference of SymbolicContext
Pull Request resolved: https://github.com/pytorch/pytorch/pull/84776
Approved by: https://github.com/AllenTiTaiWang, https://github.com/BowenBao
