[ONNX] Adjust `is_train` flag for onnx pass deduplicate initializers
Previous logic didn't consider the case for TrainingMode.PRESERVE.
A more direct way is to check `model.training`, which is the accurate
training mode, set by `exporter_context(model, training)`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/74247
Approved by: https://github.com/garymm
