Fixed size binary encoding

[eddyxu]

Problem Statement

Fixed size binary encoding is very useful when the users know that each blob has the same size, i.e., storing prepared Tensors. It allows fast point query without lookups on the offsets.

Additionally, since the data is aligned on the disk/cloud storage, it allows the "dataset loader" to read a batch of tensors (either in BCHW or BCWH form) without further transposing them in memory again.