Ensure `Transform` is pickleable. (#81707)
`Transform` is not currently pickleable if the inverse transform cache `_inv` is not `None` because `_inv` is a `weakref` which cannot be serialized by `pickle`.
The following succeeds.
```python
>>> import torch as th
>>> import pickle
>>> dist = th.distributions.TransformedDistribution(
... th.distributions.Normal(0, 1),
... [th.distributions.AffineTransform(2, 3)]
... )
>>> th.save(dist, "some-file.pt")
```
But the transformed distribution can no longer be pickled after evaluating `log_prob` (which implicitly creates `_inv`).
```python
>>> dist.log_prob(th.linspace(0, 1, 10))
>>> th.save(dist, "some-file.pt")
TypeError: cannot pickle 'weakref' object
```
This PR fixes the issue by setting `_inv` to `None` in `__getstate__`. cc @fritzo, @neerajprad
Pull Request resolved: https://github.com/pytorch/pytorch/pull/81707
Approved by: https://github.com/fritzo
