[ONNX] Re-purpose 'name' field of GraphProto (#107408)
Previously, the top level GraphProto is hardcoded with name "torch_jit", and the subgraphs "torch_jit_{count}". It does not offer any insight to the graph, but rather encodes the graph producer as jit (torchscript). This is no longer true now that the graph can also be produced from dynamo.
As a naive first step, this PR re-purposes the name, to "main_graph", and "sub_graph_{count}" respectively. More delicate processing can be done to name the subgraphs with respect to their parent node or module. This can be done as follow ups.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/107408
Approved by: https://github.com/justinchuby, https://github.com/titaiwangms
