Allow None for checkpoint (#17969)

Allow None for checkpoint (#17969)

Summary:
Currently, we cannot run a checkpointed function with None argument.

```python
out = torch.utils.checkpoint.checkpoint(run_fn, input_var, None)
```

```
  File "/home/tunz/anaconda3/envs/torchdev/lib/python3.7/site-packages/torch/utils/checkpoint.py", line 14, in detach_variable
    x = inp.detach()
AttributeError: 'NoneType' object has no attribute 'detach'
```

This PR makes checkpoint function to safely handle None argument.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/17969

Differential Revision: D14475148

Pulled By: ezyang

fbshipit-source-id: 9afe9e9aac511a6df1e1620e9ac341536890d451