Augmented calls to yaml.load to use the safe loader. (#3817)

I fixed two calls to yaml.load to explicitly use the Safe Loader. I
copied the code from other repos that do it this way.

Author: TimAtGoogle

build_tools/autogen_ltc_backend.py

@@ -30,6 +30,11 @@
 TORCHGEN_DIR = Path(torchgen.__path__[0]).resolve()
 TORCH_MLIR_DIR = Path(__file__).resolve().parent.parent
 
+# Safely load fast C Yaml loader if it is are available
+try:
+    from yaml import CSafeLoader as Loader
+except ImportError:
+    from yaml import SafeLoader as Loader #type:ignore[assignment, misc]
 
 def reindent(text, prefix=""):
     return indent(dedent(text), prefix)
@@ -175,7 +180,7 @@ def generate_native_functions(self):
         )
         ts_native_yaml = None
         if ts_native_yaml_path.exists():
-            ts_native_yaml = yaml.load(ts_native_yaml_path.read_text(), yaml.CLoader)
+            ts_native_yaml = yaml.load(ts_native_yaml_path.read_text(), Loader)
         else:
             logging.warning(
                 f"Could not find `ts_native_functions.yaml` at {ts_native_yaml_path}"
@@ -208,7 +213,7 @@ def get_opnames(ops):
         )
 
         with self.config_path.open() as f:
-            config = yaml.load(f, yaml.CLoader)
+            config = yaml.load(f, Loader)
 
         # List of unsupported ops in LTC autogen because of some error
         blacklist = set(config.get("blacklist", []))