Add helpful information to the gradient/inplace operation exception (#18523)

Add helpful information to the gradient/inplace operation exception (#18523)

Summary:
To debug a `one of the variables needed for gradient computation has been modified by an inplace operation` error, I wanted to know *which* variable has been modified, so I extended the error message with what information is easily available at this point.

Before:
```
RuntimeError: one of the variables needed for gradient computation has been modified by an inplace operation
```

After:
```
RuntimeError: one of the variables needed for gradient computation has been modified by an inplace operation: [torch.cuda.FloatTensor [80, 1]], which is output 0 of UnsqueezeBackward0, is at version 1, not expected version 0. Hint: enable anomaly detection to find the forward pass operation which modified it.
```

The hint to enable anomaly detection is only shown when it is not enabled. It's meant to save people some googling. I'd even go further and reference `torch.autograd.set_detect_anomaly(True)`, but maybe we're not running Python?

Disclaimer: I haven't looked at other parts of the code to check if using `std::stringstream` is acceptable practice, let me know if it isn't. Similarly, I haven't checked about indentation practices.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18523

Differential Revision: D14683249

Pulled By: soumith

fbshipit-source-id: f97a99d4aabea7461df766d66cd72300b48e2350