Make ExportedProgram valid tracing callable (#107657)
In this PR, we make ExportedProgram valid callable to export for re-exporting. Note that we don't allow any new constraints specified from user as we don't have any way of handling it right now. There are some caveats that is worth mentioning in this PR.
Today, graph_module.meta is not preserved (note that this is different from node level meta which we preserve). Our export logic relies on this meta to process the constraints. But if we skip dynamo, we will have to preserve the constraints stored in graph_module.meta. Once dynamo supports retracibility, we don't have to do this anymore. I currently manually save graph_module.meta at following places:
1. After ExportedProgram.module()
2. After ExportedProgram.transform()
3. At construction site of ExportedProgram.
Jerry will add the update on the quantization side as well.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/107657
Approved by: https://github.com/gmagogsfm
