(1) Directly write to file in BlobWriter::Add and destruct the MatOwner to release the rams.
(2) Write a fake header to indicate this is V2, and write correct header and directory at the end of the file.
(3) Tested on loading sbs written the old way, and new way, both worked.
PiperOrigin-RevId: 789306837
This replaces per-weight instantiations of all code with only per-MatMul/norm.
Reduces binary size by 133KiB.
WeightsOwner is no longer required for type erasing, hence it is replaced with ModelWeightsPtrs.
Also remove unused EmbedToken, replaced with EmbedMMToken.
PiperOrigin-RevId: 766497657
Jan Wassenberg
Charles Zhao