🐛 Bug

A Transformer that I used torch.save on with Pytorch 1.8.1 cannot load on Pytorch 1.9.0.

To Reproduce

Steps to reproduce the behavior:

On Pytorch 1.8.1:

import torch
from torch.nn import Transformer
torch.save(Transformer(), 'transformer.pt')

Then update Pytorch to 1.9.0

import torch
torch.load('transformer.pt')

The following stacktrace will show:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File ".../python3.8/site-packages/torch/serialization.py", line 607, in load
    return _load(opened_zipfile, map_location, pickle_module, **pickle_load_args)
  File ".../python3.8/site-packages/torch/serialization.py", line 882, in _load
    result = unpickler.load()
  File ".../python3.8/site-packages/torch/serialization.py", line 875, in find_class
    return super().find_class(mod_name, name)
AttributeError: Can't get attribute '_LinearWithBias' on <module 'torch.nn.modules.linear' from '.../python3.8/site-packages/torch/nn/modules/linear.py'>

Expected behavior

torch.save() on Pytorch X should be able to load modules that are saved on previous versions (#6801 (comment) says that torch.load is supposed to be backwards-compatible)

In this specific case, redefining the _LinearWithBias module in torch.nn.modules.linear would solve the issue.

Environment

• PyTorch Version (e.g., 1.0): 1.9.0
• OS (e.g., Linux): macOS (but it also happens on Linux with a model loaded & saved there)
• How you installed PyTorch (conda, pip, source): pip
• Python version: 3.8.7
• CUDA/cuDNN version: None
• GPU models and configuration: No

cc @mruberry