Port signal custom ops to TensorFlow and TFLite C APIs (#3880)

* Port signal custom ops to TensorFlow C API and restore BUILD files

Vendor the 16 pure-C TensorFlow headers in third_party/tf_c_headers and port
all 9 signal custom ops and kernels from tensorflow/core/framework C++ headers
to tensorflow/c/{ops,kernels}.h. Restore the Bazel BUILD files and tflm_signal.bzl
so the signal custom-op shared library plugins build without downloading
TensorFlow at build time.

BUG=n/a

* Add TFLite C API kernels for signal custom ops

Vendor the 12 pure-C TFLite Opaque API headers in third_party/tflite_c_headers
and implement all 14 signal custom ops in signal/lite/kernels using
TfLiteOperator. Each op exposes Create*Operator() for LiteRT/C API callers and
Register_*() for TFLite MutableOpResolver callers, without requiring a TFLite
repo dependency at build time.

BUG=n/a

* Fix clang-tidy warnings in signal C API ops and kernels

- Move GetFlatSize and MakeRegistration to kernel_util.cc so compile_commands.json includes -Ithird_party/tflite_c_headers for kernel_util.h.
- Replace static lambda initializers in signal/tensorflow_core with __attribute__((constructor)) functions.
- Cast integer operands before multiplication and replace long long / signed bitwise shift.

BUG=n/a

* Simplify signal C API ops and kernels with C++ RAII wrappers

Wrap TensorFlow and TFLite C API handles, status propagation, and
registration callbacks in small C++ RAII helpers so ops and kernels read
like standard C++ classes without manual memory management.

BUG=n/a

* Add sync scripts and update READMEs for C API headers

Add sync.sh to third_party/tf_c_headers and third_party/tflite_c_headers
to fetch the pure-C headers verbatim from upstream TensorFlow (pinned to
v2.20.0 by default, or from a local checkout). Document how to run the
sync scripts in both README.md files.

BUG=n/a

* Add TFLite and LiteRT registration and tests for signal custom ops

* Consolidate signal build rules in py_rules.bzl and add tflm_oss_manual_tags

* Rename OpDefBuilder to SignalOpDefBuilder and document signal ops in Python

- Rename tflite::tflm_signal::OpDefBuilder to SignalOpDefBuilder in shape_inference_utils.h and clarify that it wraps TF_OpDefinitionBuilder* from tensorflow/c/ops.h (which has no .Doc() API).

- Replace detached REGISTER_OP doc blocks in signal/tensorflow_core/ops/*.cc with uniform registration comments pointing to the Python op modules.

- Replace internal _*_wrapper closures in python/tflite_micro/signal/ops/*.py with direct top-level functions and complete Google-style docstrings on all 14 public signal ops.

BUG=n/a

Author: veblush

python/py_rules.bzl

@@ -17,17 +17,39 @@ target under the tflite_micro namespace.
 """
 
 load("@pybind11_bazel//:build_defs.bzl", _pybind_extension = "pybind_extension", _pybind_library = "pybind_library")
+load("@rules_cc//cc:cc_binary.bzl", "cc_binary")
+load("@rules_cc//cc:cc_library.bzl", "cc_library")
 load("@rules_python//python:defs.bzl", "py_binary", "py_library", "py_test")
 load("@rules_python//python:packaging.bzl", _py_package = "py_package", _py_wheel = "py_wheel")
 load("@rules_python//python:pip.bzl", _compile_pip_requirements = "compile_pip_requirements")
 load("@rules_shell//shell:sh_test.bzl", "sh_test")
+load("@tflm_pip_deps//:requirements.bzl", "requirement")
+load("//tensorflow/lite/micro:build_def.bzl", "INCOMPATIBLE_WITH_WINDOWS")
 
 compile_pip_requirements = _compile_pip_requirements
 py_package = _py_package
 py_wheel = _py_wheel
-pybind_extension = _pybind_extension
 pybind_library = _pybind_library
 
+def tflm_oss_manual_tags():
+    """Returns ["manual"] in OSS builds for targets requiring external TF/LiteRT deps."""
+    return ["manual"]
+
+def pybind_extension(name, **kwargs):
+    if "manual" in kwargs.get("tags", []):
+        _pybind_library(
+            name = name + "_pybind_lib",
+            **kwargs
+        )
+        cc_binary(
+            name = name + ".so",
+            linkshared = True,
+            tags = kwargs.get("tags", []),
+            deps = [":" + name + "_pybind_lib"],
+        )
+    else:
+        _pybind_extension(name = name, **kwargs)
+
 def tflm_py_library(deps = [], **kwargs):
     py_library(
         deps = deps + [
@@ -75,3 +97,105 @@ def tflm_whl_test(name, srcs, **kwargs):
         ],
         **kwargs
     )
+
+def py_tflm_signal_library(
+        name,
+        srcs = [],
+        deps = [],
+        visibility = None,
+        cc_op_defs = [],
+        cc_op_kernels = []):
+    """Creates build rules for signal custom ops as shared libraries."""
+    binary_path = "ops"
+    if srcs:
+        binary_path_end_pos = srcs[0].rfind("/")
+        binary_path = srcs[0][0:binary_path_end_pos]
+
+    binary_name = binary_path + "/_" + (cc_op_kernels[0][1:] if not cc_op_defs else name) + ".so"
+
+    if cc_op_defs:
+        library_name = name + "_cc"
+        cc_library(
+            name = library_name,
+            alwayslink = 1,
+            copts = ["-pthread"],
+            target_compatible_with = INCOMPATIBLE_WITH_WINDOWS,
+            deps = cc_op_defs + cc_op_kernels,
+        )
+        cc_binary(
+            name = binary_name,
+            copts = ["-pthread"],
+            linkopts = select({
+                "@platforms//os:macos": ["-Wl,-undefined,dynamic_lookup"],
+                "//conditions:default": ["-Wl,--unresolved-symbols=ignore-in-object-files"],
+            }),
+            linkshared = 1,
+            target_compatible_with = INCOMPATIBLE_WITH_WINDOWS,
+            deps = [
+                ":" + library_name,
+            ],
+        )
+
+    tflm_py_library(
+        name = name,
+        srcs = srcs,
+        data = [":" + binary_name] if cc_op_defs else [],
+        target_compatible_with = INCOMPATIBLE_WITH_WINDOWS,
+        visibility = visibility,
+        deps = deps,
+    )
+
+def tflm_signal_op_test(
+        name,
+        srcs,
+        data = [],
+        deps = [],
+        tags = [],
+        target_compatible_with = INCOMPATIBLE_WITH_WINDOWS,
+        **kwargs):
+    """Creates TFLM and TFLite test targets for a signal custom op."""
+    common_tags = tags + tflm_oss_manual_tags() + [
+        "noasan",
+        "nomsan",
+        "noubsan",
+    ]
+    tflm_py_test(
+        name = name,
+        srcs = srcs,
+        data = data,
+        tags = common_tags,
+        target_compatible_with = target_compatible_with,
+        deps = deps,
+        **kwargs
+    )
+    tflm_py_test(
+        name = name + "_tflite",
+        srcs = srcs,
+        main = srcs[0],
+        data = data + [
+            "//signal/lite/register:pywrap_signal_ops.so",
+        ],
+        env = {"TFLM_SIGNAL_USE_TFLITE": "1"},
+        tags = common_tags,
+        target_compatible_with = target_compatible_with,
+        deps = deps + [
+            requirement("ai-edge-litert"),
+        ],
+        **kwargs
+    )
+
+def tflm_signal_kernel_library(
+        name,
+        srcs = [],
+        hdrs = [],
+        deps = [],
+        copts = [],
+        alwayslink = 1):
+    cc_library(
+        name = name,
+        srcs = srcs,
+        hdrs = hdrs,
+        alwayslink = alwayslink,
+        copts = copts,
+        deps = deps,
+    )