Update on "[executorch][flat_tensor] flat tensor header"

Parse a FlatTensor header. The FlatTensor header contains:
- flatbuffer offset
- flatbuffer size
- segment base offset
- segment data size (total size of all segments)

Mostly taken from extended_header.cpp/.h

Differential Revision: [D67064570](https://our.internmc.facebook.com/intern/diff/D67064570/)

[ghstack-poisoned]

Author: lucylq

extension/flat_tensor/serialize/flat_tensor_header.h

@@ -21,7 +21,7 @@ struct FlatTensorHeader {
    * To find the header, callers should provide at least this many bytes of the
    * head of the serialized FlatTensor data.
    */
-  static constexpr size_t kNumHeadBytes = 48;
+  static constexpr size_t kNumHeadBytes = 64;
 
   /**
    * The magic bytes that identify the header. This should be in sync with

extension/flat_tensor/test/flat_tensor_header_test.cpp

@@ -39,7 +39,7 @@ constexpr char kExampleHeaderData[] = {
   // Magic bytes
   'F', 'H', '0', '1',
   // uint32_t header size (little endian)
-  0x40, 0x00, 0x00, 0x00,
+  0x28, 0x00, 0x00, 0x00,
   // uint64_t flatbuffer_offset
   0x71, 0x61, 0x51, 0x41, 0x31, 0x21, 0x11, 0x01,
   // uint64_t flatbuffer_size