[torchgen] Let native function declaration generation logic take a callable (#90780)

Retry of #90590, which is a retry of #89594. Original PR reverted due to internal breakage.
This PR fixes the breakage by adding a default value to the new argument.

This PR allows `get_native_function_declarations` API to take a function as argument. This function should take `NativeFunction` as input and emit code for native function declaration. By default it is `dest.compute_native_function_declaration`.

Pull Request resolved: https://github.com/pytorch/pytorch/pull/90780
Approved by: https://github.com/ezyang

tools/test/test_codegen.py

@@ -9,6 +9,7 @@ import torchgen.model
 import yaml
 
 from tools.autograd import gen_autograd_functions, load_derivatives
+from torchgen import dest
 from torchgen.api.types import CppSignatureGroup, DispatcherSignature
 from torchgen.context import native_function_manager
 from torchgen.gen import (
@@ -356,6 +357,7 @@ class TestGenNativeFunctionDeclaration(unittest.TestCase):
                     self.op_2_native_function,
                 ],
                 backend_indices=self.backend_indices,
+                native_function_decl_gen=dest.compute_native_function_declaration,
             )
 
     def test_native_function_declaration_1_op_1_ns_valid(self) -> None:
@@ -365,6 +367,7 @@ class TestGenNativeFunctionDeclaration(unittest.TestCase):
                 self.op_1_native_function,
             ],
             backend_indices=self.backend_indices,
+            native_function_decl_gen=dest.compute_native_function_declaration,
         )
         target = """
 namespace at {

torchgen/gen.py

@@ -5,7 +5,18 @@ import os
 import pathlib
 from collections import defaultdict, namedtuple, OrderedDict
 from dataclasses import dataclass
-from typing import Any, Dict, List, Optional, Sequence, Set, Tuple, TypeVar, Union
+from typing import (
+    Any,
+    Callable,
+    Dict,
+    List,
+    Optional,
+    Sequence,
+    Set,
+    Tuple,
+    TypeVar,
+    Union,
+)
 
 import yaml
 from typing_extensions import Literal
@@ -1406,7 +1417,17 @@ def get_native_function_declarations(
     *,
     grouped_native_functions: Sequence[Union[NativeFunction, NativeFunctionsGroup]],
     backend_indices: Dict[DispatchKey, BackendIndex],
+    native_function_decl_gen: Callable[
+        [Union[NativeFunctionsGroup, NativeFunction], BackendIndex], List[str]
+    ] = dest.compute_native_function_declaration,
 ) -> List[str]:
+    """
+    Generate kernel declarations, in `NativeFunction(s).h`.
+    :param grouped_native_functions: a sequence of `NativeFunction` or `NativeFunctionGroup`.
+    :param backend_indices: kernel collections grouped by dispatch key.
+    :param native_function_decl_gen: callable to generate kernel declaration for each `NativeFunction`.
+    :return: a list of string, from the string with all declarations, grouped by namespaces, split by newline.
+    """
     declarations: List[str] = []
     ns_grouped_kernels: Dict[str, List[str]] = defaultdict(list)
     newline = "\n"
@@ -1425,7 +1446,7 @@ def get_native_function_declarations(
                 len(native_function_namespaces) <= 1
             ), f"Codegen only supports one namespace per operator, got {native_function_namespaces} from {dispatch_keys}"
             ns_grouped_kernels[namespace].extend(
-                dest.compute_native_function_declaration(f, backend_idx)
+                native_function_decl_gen(f, backend_idx)
             )
 
     for namespace, kernels in ns_grouped_kernels.items():
@@ -1863,7 +1884,9 @@ def gen_per_operator_headers(
                 },
             )
         declarations = get_native_function_declarations(
-            grouped_native_functions=grouped_functions, backend_indices=backend_indices
+            grouped_native_functions=grouped_functions,
+            backend_indices=backend_indices,
+            native_function_decl_gen=dest.compute_native_function_declaration,
         )
         ops_fm.write_with_template(
             f"{name}_native.h",