Set _torch_version to N/A if torch is disabled.

Author: rasmi

src/diffusers/utils/import_utils.py

@@ -99,6 +99,7 @@ def _is_package_available(pkg_name: str, get_dist_name: bool = False) -> Tuple[b
 else:
     logger.info("Disabling PyTorch because USE_TORCH is set")
     _torch_available = False
+    _torch_version = "N/A"
 
 _jax_version = "N/A"
 _flax_version = "N/A"