What kind of error do you get when trying to load the .pkl file?
I’m not sure if the files really differ, since only the file extension is using another naming. You could also save a state_dict using any custom extension as seen here, which won’t change the underlying data structure:
lin = nn.Linear(1, 1)
torch.save(lin.state_dict(), "lin.my_custom_extension_format")

lin = nn.Linear(1, 1)
lin.load_state_dict(torch.load('lin.my_custom_extension_format'))
> <All keys matched successfully>