[Specialized Kernel] Update yaml syntax to use kernel instead of dispatch

test/edge/custom_ops.yaml

@@ -1,3 +1,4 @@
 - func: custom::add_3.out(Tensor a, Tensor b, Tensor c, *, Tensor(a!) out) -> Tensor(a!)
-  dispatch:
-    CPU: custom::add_3_out
+  kernels:
+    - arg_meta: null
+      kernel_name: custom::add_3_out

torchgen/executorch/parse.py

@@ -130,9 +130,7 @@ def parse_et_yaml(
     et_kernel = extract_kernel_fields(es)
 
     # Remove ET specific fields from entries for BC compatibility
-    for entry in es:
-        for field in ET_FIELDS:
-            entry.pop(field, None)
+    strip_et_fields(es)
 
     native_yaml = parse_native_yaml(
         path,
@@ -142,3 +140,12 @@ def parse_et_yaml(
         loaded_yaml=es,
     )
     return native_yaml.native_functions, et_kernel
+
+
+def strip_et_fields(es: object) -> None:
+    """Given a loaded yaml representing a list of operators,
+    remove ET specific fields from every entries for BC compatibility
+    """
+    for entry in es:  # type: ignore[attr-defined]
+        for field in ET_FIELDS:
+            entry.pop(field, None)