Add NamedData to flat_tensor schema

This allows opaque data blobs inside PTD files.

Differential Revision: [D70530888](https://our.internmc.facebook.com/intern/diff/D70530888/)

ghstack-source-id: 270932272
Pull Request resolved: #9123

Author: lucylq

extension/flat_tensor/serialize/flat_tensor.fbs

@@ -35,8 +35,8 @@ table TensorMetadata {
   // To retrieve a given tensor:
   // 1. segment_base_offset: from the file header.
   // 2. segment_offset: segments[segment_index].offset
-  // 3. tensor_offset: segments[segment_offset].tensor_metadata[j].offset
-  //    Find the relevant index j by matching on tensor fqn.
+  // 3. tensor_offset: the offset within the segment. If there is only one item
+  // in the segment, offset=0.
   offset: uint64;
 }
 
@@ -55,6 +55,15 @@ table DataSegment {
   size: uint64;
 }
 
+// Attributes a name to data referenced by FlatTensor.segments.
+table NamedData {
+  // The unique id of the data blob.
+  key: string;
+
+  // Index of the segment in FlatTensor.segments.
+  segment_index: uint32;
+}
+
 // FlatTensor is a flatbuffer-based format for storing and loading tensors.
 table FlatTensor {
   // Schema version.
@@ -70,6 +79,10 @@ table FlatTensor {
   // List of data segments that follow the FlatTensor data in this file, sorted by
   // offset. Elements in this schema can refer to these segments by index.
   segments: [DataSegment];
+
+  // List of blobs keyed by a unique name. Note that multiple 'NamedData'
+  // entries could point to the same segment index.
+  named_data: [NamedData];
 }
 
 root_type FlatTensor;

extension/flat_tensor/serialize/flat_tensor_schema.py

@@ -31,9 +31,16 @@ class DataSegment:
     size: int
 
 
+@dataclass
+class NamedData:
+    key: str
+    segment_index: int
+
+
 @dataclass
 class FlatTensor:
     version: int
     tensor_alignment: int
     tensors: List[TensorMetadata]
     segments: List[DataSegment]
+    named_data: List[NamedData]

extension/flat_tensor/serialize/serialize.py

@@ -282,6 +282,7 @@ def serialize(
             tensor_alignment=self.config.tensor_alignment,
             tensors=flat_tensor_metadata,
             segments=[DataSegment(offset=0, size=len(flat_tensor_data))],
+            named_data=[],
         )
 
         flatbuffer_payload = _serialize_to_flatbuffer(flat_tensor)