[executorch][flat_tensor] Generate flatc files

1. Move schema files under flat_tensor/serialize
2. Add targets to generate schema files

Differential Revision: [D66903492](https://our.internmc.facebook.com/intern/diff/D66903492/)

[ghstack-poisoned]

Author: lucylq

extension/flat_tensor/serialize/TARGETS

@@ -0,0 +1,16 @@
+load("@fbsource//xplat/executorch/build:runtime_wrapper.bzl", "runtime")
+load(":targets.bzl", "define_common_targets")
+
+oncall("executorch")
+
+define_common_targets()
+
+runtime.python_library(
+    name = "schema",
+    srcs = [
+        "flat_tensor_schema.py",
+    ],
+    visibility = [
+        "//executorch/...",
+    ],
+)

extension/flat_tensor/flat_tensor.fbs renamed to extension/flat_tensor/serialize/flat_tensor.fbs

@@ -0,0 +1,36 @@
+load("@fbsource//xplat/executorch/build:runtime_wrapper.bzl", "runtime")
+
+def define_common_targets():
+    runtime.genrule(
+        name = "gen_schema",
+        srcs = [
+            "flat_tensor.fbs",
+            "scalar_type.fbs",
+        ],
+        outs = {
+            "schema_generated.h": ["flat_tensor_generated.h"],
+            "scalar_type_generated.h": ["scalar_type_generated.h"]
+        },
+        cmd = " ".join([
+            "$(exe {})".format(runtime.external_dep_location("flatc")),
+            "--cpp",
+            "--cpp-std c++11",
+            "--scoped-enums",
+            "-o ${OUT}",
+            "${SRCS}",
+        ]),
+        default_outs = ["."],
+    )
+
+    runtime.cxx_library(
+        name = "generated_headers",
+        srcs = [],
+        visibility = [
+            "//executorch/...",
+        ],
+        exported_headers = {
+            "schema_generated.h": ":gen_schema[schema_generated.h]",
+            "scalar_type_generated.h": ":gen_schema[scalar_type_generated.h]",
+        },
+        exported_external_deps = ["flatbuffers-api"],
+    )