Rename tools/codegen to torchgen (#76275)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/76275

In preparation for addressing
https://github.com/pytorch/pytorch/issues/73212

Diff was generated with:

```
git mv tools/codegen torchgen
git grep -l 'tools.codegen' | xargs sed -i 's/tools.codegen/torchgen/g'
sed -i "s/\${TOOLS_PATH}\/codegen/\${TORCH_ROOT}\/torchgen/g" caffe2/CMakeLists.txt
```

and a manual edits to:

* tools/test/test_gen_backend_stubs.py
* torchgen/build.bzl
* torchgen/gen_backend_stubs.py

aka this diff:

```
 diff --git a/tools/test/test_gen_backend_stubs.py b/tools/test/test_gen_backend_stubs.py
index 3dc26c6d2d..104054575e 100644
 --- a/tools/test/test_gen_backend_stubs.py
+++ b/tools/test/test_gen_backend_stubs.py
@@ -9,7 +9,7 @@ from torchgen.gen_backend_stubs import run
 from torchgen.gen import _GLOBAL_PARSE_NATIVE_YAML_CACHE  # noqa: F401

 path = os.path.dirname(os.path.realpath(__file__))
-gen_backend_stubs_path = os.path.join(path, '../torchgen/gen_backend_stubs.py')
+gen_backend_stubs_path = os.path.join(path, '../../torchgen/gen_backend_stubs.py')

 # gen_backend_stubs.py is an integration point that is called directly by external backends.
 # The tests here are to confirm that badly formed inputs result in reasonable error messages.
 diff --git a/torchgen/build.bzl b/torchgen/build.bzl
index ed04e35a43..d00078a3cf 100644
 --- a/torchgen/build.bzl
+++ b/torchgen/build.bzl
@@ -1,6 +1,6 @@
 def define_targets(rules):
     rules.py_library(
-        name = "codegen",
+        name = "torchgen",
         srcs = rules.glob(["**/*.py"]),
         deps = [
             rules.requirement("PyYAML"),
@@ -11,6 +11,6 @@ def define_targets(rules):

     rules.py_binary(
         name = "gen",
-        srcs = [":codegen"],
+        srcs = [":torchgen"],
         visibility = ["//visibility:public"],
     )
 diff --git a/torchgen/gen_backend_stubs.py b/torchgen/gen_backend_stubs.py
index c1a672a655..beee7a15e0 100644
 --- a/torchgen/gen_backend_stubs.py
+++ b/torchgen/gen_backend_stubs.py
@@ -474,7 +474,7 @@ def run(
 ) -> None:

     # Assumes that this file lives at PYTORCH_ROOT/torchgen/gen_backend_stubs.py
-    pytorch_root = pathlib.Path(__file__).parent.parent.parent.absolute()
+    pytorch_root = pathlib.Path(__file__).parent.parent.absolute()
     template_dir = os.path.join(pytorch_root, "aten/src/ATen/templates")

     def make_file_manager(install_dir: str) -> FileManager:
```

run_all_fbandroid_tests

Test Plan: sandcastle

Reviewed By: albanD, ngimel

Differential Revision: D35770317

fbshipit-source-id: 153ac4a7fef15b1e750812a90bfafdbc8f1ebcdf
(cherry picked from commit c6d485d1d4648fa1c8a4c14c5bf3d8e899b9b4dd)

tools/jit/gen_unboxing.py

@@ -3,15 +3,15 @@ import argparse
 import os
 import pathlib
 from dataclasses import dataclass
-from tools.codegen.api import unboxing
-from tools.codegen.api.translate import translate
-from tools.codegen.api.types import CppSignatureGroup
-from tools.codegen.api.unboxing import convert_arguments
-from tools.codegen.context import method_with_native_function
-from tools.codegen.gen import parse_native_yaml, cpp_string
-from tools.codegen.model import NativeFunction, NativeFunctionsGroup, Variant
-from tools.codegen.selective_build.selector import SelectiveBuilder
-from tools.codegen.utils import Target, FileManager, mapMaybe, make_file_manager
+from torchgen.api import unboxing
+from torchgen.api.translate import translate
+from torchgen.api.types import CppSignatureGroup
+from torchgen.api.unboxing import convert_arguments
+from torchgen.context import method_with_native_function
+from torchgen.gen import parse_native_yaml, cpp_string
+from torchgen.model import NativeFunction, NativeFunctionsGroup, Variant
+from torchgen.selective_build.selector import SelectiveBuilder
+from torchgen.utils import Target, FileManager, mapMaybe, make_file_manager
 from typing import Union, Sequence
 from typing_extensions import Literal