Add warning if tensor cores are not used (#88844)
Fixes https://github.com/pytorch/torchdynamo/issues/1839
Should I do this for all backends or just inductor?
## Test
On a V100 I got from AWS
```python
from torch._dynamo import optimize
import torch
def fn(x, y):
a = torch.cos(x)
b = torch.sin(y)
return a + b
new_fn = optimize("inductor")(fn)
a = new_fn(torch.Tensor(1),torch.Tensor(1))
print(a)
```
## New logs
```
(sourcetorch) ubuntu@ip-172-31-31-152:~/test$ python test.py
/home/ubuntu/pytorch/torch/_dynamo/eval_frame.py:318: UserWarning: Tensor cores are available but not enabled. Consider setting torch.backends.cuda.matmul.allow_tf32 == True in your python script for speedups
warnings.warn(
tensor([1.3717])
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/88844
Approved by: https://github.com/ngimel, https://github.com/mlazos, https://github.com/anijain2305
