Fix compatibility problem with LSTMs and torch.save (#57558)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/57558
Fixes #53359
If someone directly saves an nn.LSTM in PyTorch 1.7 and then loads it in PyTorch
1.8, it errors out with the following:
```
(In PyTorch 1.7)
import torch
model = torch.nn.LSTM(2, 3)
torch.save(model, 'lstm17.pt')
(In PyTorch 1.8)
model = torch.load('lstm17.pt')
AttributeError: 'LSTM' object has no attribute 'proj_size'
```
Although we do not officially support this (directly saving modules via
torch.save), it used to work and the fix is very simple. This PR adds an
extra line to `__setstate__`: if the state we are passed does not have
a `proj_size` attribute, we assume it was saved from PyTorch 1.7 and
older and set `proj_size` equal to 0.
Test Plan:
I wrote a test that tests `__setstate__`. But also,
Run the following:
```
(In PyTorch 1.7)
import torch
x = torch.ones(32, 5, 2)
model = torch.nn.LSTM(2, 3)
torch.save(model, 'lstm17.pt')
y17 = model(x)
(Using this PR)
model = torch.load('lstm17.pt')
x = torch.ones(32, 5, 2)
y18 = model(x)
```
and finally compare y17 and y18.
Reviewed By: mrshenli
Differential Revision: D28198477
Pulled By: zou3519
fbshipit-source-id: e107d1ebdda23a195a1c3574de32a444eeb16191
