Update on "Add segment_data_size to extended header"

Store the segment data size. Allows us to calculate the total PTE file size and error out if the full PTE file was not downloaded.

Next diff: check file size at runtime.

Differential Revision: [D81829663](https://our.internmc.facebook.com/intern/diff/D81829663/)

[ghstack-poisoned]

Author: lucylq

docs/source/pte-file-format.md

@@ -72,7 +72,7 @@ Optional extended header:
 | [24..31] uint64_t offset (from byte offset zero above) to the start of the
 |          first segment, or zero if there are no segments.
 | [32..39] uint64_t size of the segment data, ie. the size from the segment_base_offset
-|          to the end of the file
+|          to the end of the segments.
 | [40..?]  Any zero-padding necessary to preserve the alignment of the data
 |          that follows.
 End of optional extended header.

exir/_serialize/_program.py

@@ -157,7 +157,8 @@ class _ExtendedHeader:
     # Offset to the start of the first segment, or zero if there
     # are no segments.
     segment_base_offset: int
-    # Size of the segment data, in bytes, or zero if there are no segments.
+    # Size of the segment data, in bytes, or zero if there are no segments, or
+    # if the this field isn't populated in the PTE file.
     segment_data_size: int
 
     # The magic bytes read from or to be written to the binary header.