Check for mutable default parameters (#29833)
Summary:
Fix for https://github.com/pytorch/pytorch/issues/21545
We we were silently giving wrong semantics previously:
Python behavior:
```
def test(x=[]):
x.append(1)
return len(x)
print(test()) # 1
print(test()) # 2
```
By checking at the python layer, we prevent any new models from serializing this behavior but do not break existing serialized models.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/29833
Differential Revision: D18513168
Pulled By: eellison
fbshipit-source-id: 6fe73f28e1f9d39dedeaf67a04718089d14401a1
