Quantize the whole decoder in the int8 variant

The int8 build quantized only the tied logit head, leaving the rest of the
decoder in fp32. Quantizing every decode-side linear takes the file from
154 MB to 130 MB with a transcript identical to the fp32 decoder.

Encode is untouched and its output is bit-identical to the fp16 variant:
the cross-attention key and value projections are excluded, since the
encoder hoists them and they run in the Vulkan partition.

vulkan/whisper_tiny_vulkan_int8.pte

@@ -1,3 +1,3 @@
 version https://git-lfs.github.com/spec/v1
-oid sha256:a9014d2ceb0c6a5bafe94086274556b11575d37840b2ae9d4d9aec31b90821de
-size 154213348
+oid sha256:1953ba74947ee23d5f076fa8a198011bd0d6213e5a5e615ee4696ebb418fe20a
+size 129521380