[torchgen] reference generated comment to actual location of the generator and template (#130020)

As per title.

```diff
# torch/_VF.pyi

- # @generated from torch/_C/_VariableFunctions.pyi.in
+ # @generated by tools/pyi/gen_pyi.py from torch/_C/_VariableFunctions.pyi.in
```

```diff
# torch/return_types.pyi

- # @generated from torch/_C/return_types.pyi
+ # @generated by tools/pyi/gen_pyi.py from torch/_C/return_types.pyi.in
```

```diff
# torch/_C/__init__.pyi

- # @generated from torch/_C/__init__.pyi.in
+ # @generated by tools/pyi/gen_pyi.py from torch/_C/__init__.pyi.in
```

```diff
# torch/_C/_nn.pyi

+ # @generated by tools/pyi/gen_pyi.py from torch/_C/_nn.pyi.in
```

```diff
# torch/_C/_VariableFunctions.pyi

- # @generated from torch/_C/_VariableFunctions.pyi.in
+ # @generated by tools/pyi/gen_pyi.py from torch/_C/_VariableFunctions.pyi.in
```

```diff
# torch/nn/functional.pyi

+ # @generated by tools/pyi/gen_pyi.py from torch/nn/functional.pyi.in
```

Pull Request resolved: https://github.com/pytorch/pytorch/pull/130020
Approved by: https://github.com/ezyang

tools/pyi/gen_pyi.py

@@ -1426,36 +1426,22 @@ def gen_pyi(
     fm.write_with_template(
         "torch/_C/__init__.pyi",
         "torch/_C/__init__.pyi.in",
-        lambda: {
-            "generated_comment": "@" + "generated from torch/_C/__init__.pyi.in",
-            **env,
-        },
+        lambda: env,
     )
     fm.write_with_template(
         "torch/_C/_VariableFunctions.pyi",
         "torch/_C/_VariableFunctions.pyi.in",
-        lambda: {
-            "generated_comment": "@"
-            + "generated from torch/_C/_VariableFunctions.pyi.in",
-            **env,
-        },
+        lambda: env,
     )
     fm.write_with_template(
         "torch/_VF.pyi",
         "torch/_C/_VariableFunctions.pyi.in",
-        lambda: {
-            "generated_comment": "@"
-            + "generated from torch/_C/_VariableFunctions.pyi.in",
-            **env,
-        },
+        lambda: env,
     )
     fm.write_with_template(
         "torch/return_types.pyi",
         "torch/_C/return_types.pyi.in",
-        lambda: {
-            "generated_comment": "@" + "generated from torch/_C/return_types.pyi",
-            **env,
-        },
+        lambda: env,
     )
     gen_nn_functional(fm)
 

torch/_C/_nn.pyi.in

@@ -1,4 +1,6 @@
+# ${generated_comment}
 # mypy: disable-error-code="type-arg"
+
 from typing import List, Optional, overload, Sequence, Tuple, Union
 
 from torch import memory_format, Tensor

torch/_C/return_types.pyi.in

@@ -1,5 +1,5 @@
-# mypy: allow-untyped-defs
 # ${generated_comment}
+# mypy: allow-untyped-defs
 
 from typing import (
     Any,

torch/nn/functional.pyi.in

@@ -1,4 +1,6 @@
+# ${generated_comment}
 # mypy: allow-untyped-defs
+
 from typing import (
     Any,
     Callable,

torchgen/utils.py

@@ -9,6 +9,7 @@ import sys
 import textwrap
 from dataclasses import fields, is_dataclass
 from enum import auto, Enum
+from pathlib import Path
 from typing import (
     Any,
     Callable,
@@ -30,6 +31,9 @@ if TYPE_CHECKING:
     from argparse import Namespace
 
 
+REPO_ROOT = Path(__file__).absolute().parent.parent
+
+
 # Many of these functions share logic for defining both the definition
 # and declaration (for example, the function signature is the same), so
 # we organize them into one function that takes a Target to say which
@@ -155,11 +159,26 @@ class FileManager:
         template_path = os.path.join(self.template_dir, template_fn)
         env = env_callable()
         if isinstance(env, dict):
-            # TODO: Update the comment reference to the correct location
             if "generated_comment" not in env:
-                comment = "@" + "generated by torchgen/gen.py"
-                comment += f" from {os.path.basename(template_path)}"
-                env["generated_comment"] = comment
+                generator_default = REPO_ROOT / "torchgen" / "gen.py"
+                try:
+                    generator = Path(
+                        sys.modules["__main__"].__file__ or generator_default
+                    ).absolute()
+                except (KeyError, AttributeError):
+                    generator = generator_default.absolute()
+
+                try:
+                    generator_path = generator.relative_to(REPO_ROOT).as_posix()
+                except ValueError:
+                    generator_path = generator.name
+
+                env = {
+                    **env,  # copy the original dict instead of mutating it
+                    "generated_comment": (
+                        "@" + f"generated by {generator_path} from {template_fn}"
+                    ),
+                }
             template = _read_template(template_path)
             return template.substitute(env)
         elif isinstance(env, str):