use torch.compiler.is_compiling (#739)

following
```
/path/to/ao/torchao/float8/float8_linear_utils.py:165: FutureWarning: `torch._dynamo.external_utils.is_compiling` is deprecated. Use `torch.compiler.is_compiling` instead.
  if not torch._dynamo.is_compiling():
```

Signed-off-by: Masaki Kozuki <mkozuki@nvidia.com>

torchao/float8/float8_linear_utils.py

@@ -162,7 +162,7 @@ def get_float8_layers(model: torch.nn.Module):
 
  # Get all fp8 layers and tensors
  fp8_layers = [child for child in model.modules() if isinstance(child, Float8Linear)]
- if not torch._dynamo.is_compiling():
+ if not torch.compiler.is_compiling():
  for layer in fp8_layers:
  for buf in layer.buffers():
  torch._dynamo.mark_static_address(buf, guard=True)