Add conversions of U8 to/from BF16 and FP8

Adds type to PadAttribute and ClampAttribute so their pad_const
and max_val/min_val can be deserialized according to type

Adds conversion functions of U8 arrays to/from BF16/FP8 values
Also, refactor and expose TosaSerializer.convertDataToUint8Vec
for converting dtype/data to uint8 list for serialization
And modify convertDataToUint8Vec to serialize bf16 values into
2 bytes each, and serialize fp8 values into single bytes each.

Signed-off-by: Tai Ly <tai.ly@arm.com>
Change-Id: I05659e8187c76d359f1cc9f71c8c23cafd0e877f
diff --git a/schema/tosa.fbs b/schema/tosa.fbs
index 79b83b1..7b5948b 100644
--- a/schema/tosa.fbs
+++ b/schema/tosa.fbs
@@ -185,6 +185,7 @@
 
 table PadAttribute {
   pad_const: [ubyte] (force_align: 8);
+  type: DType;
 }
 
 table AxisAttribute {
@@ -201,6 +202,7 @@
 table ClampAttribute {
   min_val: [ubyte] (force_align: 8);
   max_val: [ubyte] (force_align: 8);
+  type: DType;
 }
 
 table RescaleAttribute {