Fix initializer naming at torch.onnx.ExportOutput.save_model_with_external_data (#105002)
This PR is only relevant for the Fake tensor Mode ONNX export. For the conventional export, everything is unchanged.
* An optional `rename_initializer=False` argument is added to an internal function `torch/onnx/_internal/fx/serialization.py::save_model_with_external_data` which is used by the public API `ExportOutput.save`.
* The default behavior (`rename_initializer=False`) is meant to be used by public API `torch.onnx.dynamo_export` with the default Dynamo-based FX tracer (`DynamoExport`). In this scenario, both graph ONNX graph inputs and initializers have matching name with `.` in it (e.g. `linear.weight`)
* `rename_initializer=True` is meant to be used by `torch.onnx.dynamo_export` with a non-publicly-supported FX tracer called `FXSymbolicTracer`. This tracer lifts the FX graph initializers as inputs before FX->ONNX start, and because of this, the initializer names must be valid python identifiers (meaning `.` are not supported argument name and must be replaced by `_` or similar). This causes the graph inputs to have names with `_` (e.g. `linear_weight`) while the initializers have `.` (e.g. `linear.weight`) in their name. This flag resolves this mismatch by replacing `.` by `_` when saving the ONNX proto (`save_model_with_external_data`).
* This PR also adds unit tests for numerical validation against pytorch eager for onnx export using dynamo-based fx tracer and fake mode enabled. (There are already tests for export with fx symbolic tracer with fake mode)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/105002
Approved by: https://github.com/BowenBao
