[executorch][serialization] Refactor flatbuffer utils into separate file

Pull Request resolved: #7254

Todo: let xnnpack and vulkan serialization use these utils instead of redefining the same functions.

For usage in extension/flat_tensor/serialize.
ghstack-source-id: 257299390
@exported-using-ghexport

Differential Revision: [D66854756](https://our.internmc.facebook.com/intern/diff/D66854756/)

Author: lucylq

exir/_serialize/TARGETS

@@ -33,6 +33,7 @@ runtime.python_library(
         "_dataclass.py",
         "_flatbuffer.py",
         "_program.py",
+        "utils.py",
     ],
     resources = {
         "//executorch/schema:program.fbs": "program.fbs",

exir/_serialize/_program.py

@@ -11,7 +11,7 @@
 import re
 
 from dataclasses import dataclass
-from typing import ClassVar, List, Literal, Optional, Tuple
+from typing import ClassVar, List, Optional, Tuple
 
 from executorch.exir._serialize._cord import Cord
 from executorch.exir._serialize._dataclass import _DataclassEncoder, _json_to_dataclass
@@ -21,6 +21,13 @@
     _program_json_to_flatbuffer,
 )
 
+from executorch.exir._serialize.utils import (
+    _aligned_size,
+    _HEADER_BYTEORDER,
+    _pad_to,
+    _padding_required,
+)
+
 from executorch.exir.schema import (
     BackendDelegateDataReference,
     BackendDelegateInlineData,
@@ -33,12 +40,6 @@
 from executorch.exir.tensor import ALIGNMENT
 
 
-# Byte order of numbers written to program headers. Always little-endian
-# regardless of the host system, since all commonly-used modern CPUs are little
-# endian.
-_HEADER_BYTEORDER: Literal["little"] = "little"
-
-
 def _program_to_json(program: Program) -> str:
     """Returns the JSON representation of the given Program."""
     return json.dumps(program, cls=_DataclassEncoder)
@@ -50,19 +51,6 @@ def _json_to_program(program_json: bytes) -> Program:
     return _json_to_dataclass(json.loads(program_json), cls=Program)
 
 
-def _padding_required(offset: int, alignment: int) -> int:
-    """Returns the padding required to align `offset` to `alignment`."""
-    remainder: int = offset % alignment
-    if remainder != 0:
-        return alignment - remainder
-    return 0
-
-
-def _aligned_size(input_size: int, alignment: int) -> int:
-    """Returns input_size padded up to the next whole multiple of alignment."""
-    return input_size + _padding_required(input_size, alignment)
-
-
 def _insert_flatbuffer_header(
     flatbuffer_data: bytes, magic_regex: str, header_data: bytes
 ) -> bytes:
@@ -211,25 +199,6 @@ def to_bytes(self) -> bytes:
         return data
 
 
-def _pad_to(data: bytes, length: int) -> bytes:
-    """Returns the input followed by enough zero bytes to become the requested length.
-
-    Args:
-        data: The data to pad.
-        length: The length of the returned data.
-    Returns:
-        The padded data.
-    Raises:
-        ValueError: If the requested length is less than the input length.
-    """
-    if length < len(data):
-        raise ValueError(f"Data length {len(data)} > padded length {length}")
-    if length > len(data):
-        data = data + b"\x00" * (length - len(data))
-    assert len(data) == length
-    return data
-
-
 def _get_extended_header(program_data: bytes) -> Optional[_ExtendedHeader]:
     """Returns the extended header of the program data, if present and valid."""
     try:

exir/_serialize/utils.py

@@ -0,0 +1,42 @@
+# (c) Meta Platforms, Inc. and affiliates. Confidential and proprietary.
+
+# pyre-strict
+
+from typing import Literal
+
+# Byte order of numbers written to program headers. Always little-endian
+# regardless of the host system, since all commonly-used modern CPUs are little
+# endian.
+_HEADER_BYTEORDER: Literal["little"] = "little"
+
+
+def _pad_to(data: bytes, length: int) -> bytes:
+    """Returns the input followed by enough zero bytes to become the requested length.
+
+    Args:
+        data: The data to pad.
+        length: The length of the returned data.
+    Returns:
+        The padded data.
+    Raises:
+        ValueError: If the requested length is less than the input length.
+    """
+    if length < len(data):
+        raise ValueError(f"Data length {len(data)} > padded length {length}")
+    if length > len(data):
+        data = data + b"\x00" * (length - len(data))
+    assert len(data) == length
+    return data
+
+
+def _padding_required(offset: int, alignment: int) -> int:
+    """Returns the padding required to align `offset` to `alignment`."""
+    remainder: int = offset % alignment
+    if remainder != 0:
+        return alignment - remainder
+    return 0
+
+
+def _aligned_size(input_size: int, alignment: int) -> int:
+    """Returns input_size padded up to the next whole multiple of alignment."""
+    return input_size + _padding_required(input_size, alignment)