Issue warning with reference to user code rather than torch (pytorch#155112)

Re-raising of pytorch#129959 as that was closed.

Warning message before:
```
/home/admin/.local/share/hatch/env/virtual/toms-project-1/Qv9k_r_5/dev/lib/python3.10/site-packages/torch/cuda/amp/grad_scaler.py:120: UserWarning: torch.cuda.amp.GradScaler is enabled, but CUDA is not available.  Disabling.
```

Warning message after:
```
/path/to/my/code:91: UserWarning: torch.cuda.amp.GradScaler is enabled, but CUDA is not available.  Disabling.
```

Helps the user find where the issue stems from in their code. What do you think?

(Looks like "skip_file_prefixes" is not available until Python 3.12 minimum...)

Pull Request resolved: pytorch#155112
Approved by: https://github.com/Skylion007, https://github.com/cyyever

Author: tmct

torch/amp/grad_scaler.py

@@ -134,7 +134,8 @@ def __init__(
         if self._device == "cuda":
             if enabled and torch.cuda.amp.common.amp_definitely_not_available():
                 warnings.warn(
-                    "torch.cuda.amp.GradScaler is enabled, but CUDA is not available.  Disabling."
+                    "torch.cuda.amp.GradScaler is enabled, but CUDA is not available.  Disabling.",
+                    stacklevel=2,
                 )
                 self._enabled = False