Based on your latest debugging, the issue might be related to QAT.
I’m not deeply familiar with QAT and the support to use torch.save on the model directly.
However, generally I would not recommend to save the model directly, as it can break in various ways.
The better way would be to save the state_dict (and additional configs etc.). When reloading you would then recreate the model object and use load_state_dict.