Fix bug in fp8e5m2 serialisation

Previously we were incorrectly serialising fp8e5m2 tensors into the
flatbuffers. Now we handle serialisation properly.

The code still assumes the input array to be of dtype uint8. A future
patch should allow fp8e5m2 tensors to be passed in native form, with the
library itself handling conversions to uint8 if necessary.

Signed-off-by: Ian Tayler Lessa <ian.taylerlessa@arm.com>
Change-Id: Ib4772a7ae01d5ce5e85f426dec531a5f4778e2f0