[executorch][serialization] Move DataSegment into shared common.fbs

So that `DataSegment` can be shared by incoming data.fbs.

Differential Revision: [D65434369](https://our.internmc.facebook.com/intern/diff/D65434369/)

[ghstack-poisoned]

exir/common_schema.py

@@ -6,9 +6,12 @@
 
 # pyre-unsafe
 
+from dataclasses import dataclass
 from enum import IntEnum
 
 
+# Check exir/common.fbs for explanations of these fields.
+
 class ScalarType(IntEnum):
     BYTE = 0
     CHAR = 1
@@ -29,3 +32,9 @@ class ScalarType(IntEnum):
     QUINT4x2 = 16
     QUINT2x4 = 17
     BITS16 = 22
+
+
+@dataclass
+class DataSegment:
+    offset: int
+    size: int

exir/schema.py

@@ -12,7 +12,7 @@
 
 from executorch.exir.backend.compile_spec_schema import CompileSpec
 
-from executorch.exir.common_schema import ScalarType
+from executorch.exir.common_schema import ScalarType, DataSegment
 
 
 @dataclass
@@ -272,12 +272,6 @@ class ExecutionPlan:
     non_const_buffer_sizes: List[int]
 
 
-@dataclass
-class DataSegment:
-    offset: int
-    size: int
-
-
 @dataclass
 class SubsegmentOffsets:
     segment_index: int

schema/common.fbs

@@ -35,3 +35,19 @@ enum ScalarType : byte {
   // BITS4x2 = 20,
   // BITS8 = 21,
 }
+
+// Describes a contiguous piece of data that lives outside of the flatbuffer data,
+// typically appended afterwards in the file.
+// For .pte and .ptd files, the "extended header" in the file, when present,
+// points to the segment base offset.
+table DataSegment {
+  // Segment offsets are relative to the segment base offset provided in the
+  // extended file header. Segments will typically be aligned in a way to make
+  // it possible to use mmap() to load them.
+  offset: uint64;
+
+  // The size in bytes of valid data starting at the offset. The segment
+  // data may be followed by padding before the segment that follows it,
+  // to make it easier to use mmap().
+  size: uint64;
+}

schema/program.fbs

@@ -392,21 +392,6 @@ table BackendDelegateInlineData {
   data: [ubyte] (force_align: 16);  // @executorch-delegate-alignment
 }
 
-// Describes a contiguous piece of data that lives outside of the flatbuffer data,
-// typically appended afterwards in the file. The "extended header" in the file,
-// when present, points to the segment base offset.
-table DataSegment {
-  // Segment offsets are relative to the segment base offset provided in
-  // the extended file header. Segments will typically be aligned in a
-  // way to make it possible to use mmap() to load them.
-  offset: uint64;
-
-  // The size in bytes of valid data starting at the offset. The segment
-  // data may be followed by padding before the segment that follows it,
-  // to make it easier to use mmap().
-  size: uint64;
-}
-
 // Describes data offsets into a particular segment
 table SubsegmentOffsets {
   // Index of the segment in Program.segments