Enable framework generator to save numpy files in native dtypes

With the following patch merged, the tosa serialization_lib is
able to read u/int8, u/int16 in their native dtypes:
https://review.mlplatform.org/c/tosa/serialization_lib/+/9871



Signed-off-by: Jerry Ge <jerry.ge@arm.com>
Change-Id: I5e63fb70b055cc5ee33e7dc26237eef9bedeac30