[CUDA][CUDA Graphs] Fix CUDAGraph::reset function (#108896)
The following two cases fail due to a small oversight `CUDAGraph::reset()` that causes failures in graph destructor
```Python
import torch
x = torch.zeros(4, device="cuda")
g = torch.cuda.CUDAGraph()
with torch.cuda.graph(g):
x = x + 1
g.reset()
del g
```
that fails with:
```
terminate called after throwing an instance of 'c10::Error'
what(): uc >= 0 INTERNAL ASSERT FAILED at ".../pytorch/c10/cuda/CUDACachingAllocator.cpp":2157, please report a bug to PyTorch.
```
and reset and subsequent re-capture
```Python
import torch
x = torch.zeros(4, device="cuda")
g = torch.cuda.CUDAGraph()
with torch.cuda.graph(g):
x = x + 1
g.reset()
with torch.cuda.graph(g):
x = x + 1
g.replay()
```
which fails with:
```
Traceback (most recent call last):
File "test_graph.py", line 11, in <module>
with torch.cuda.graph(g):
File ".../pytorch/torch/cuda/graphs.py", line 192, in __enter__
self.cuda_graph.capture_begin(
File ".../pytorch/torch/cuda/graphs.py", line 77, in capture_begin
super().capture_begin(pool=pool, capture_error_mode=capture_error_mode)
RuntimeError: This CUDAGraph instance already owns a captured graph. To capture a new graph, create a new instance.
```
This PR fixes `CUDAGraph::reset()` function for above to use cases.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/108896
Approved by: https://github.com/ezyang
