[CUDA graphs] Add warning if captured graph is empty (#88754)
Fixes #87894
This PR adds a warning if captured graph is empty (consists of zero nodes).
The example snippet where would it be useful:
```python
import torch
x = torch.randn(10)
z = torch.zeros(10)
g = torch.cuda.CUDAGraph()
with torch.cuda.graph(g):
z = x * x
# Warn user
```
and in #87894
Pull Request resolved: https://github.com/pytorch/pytorch/pull/88754
Approved by: https://github.com/ezyang
