Increment start_pos by encoded size in generate #2418

Job	Run time
test-readme-any / linux-job	11m 41s
test-advanced-any / linux-job	17m 38s
test-advanced-cpu / linux-job	17m 43s
test-gguf-any / linux-job	17m 34s
test-quantization-any / linux-job	17m 49s
test-quantization-cpu / linux-job	18m 0s
test-readme-cpu / linux-job	11m 32s
test-evaluation-any / linux-job	17m 47s
test-evaluation-cpu / linux-job	17m 51s
test-gguf-cpu / linux-job	17m 34s
test-multimodal-any / linux-job	4m 17s
test-multimodal-cpu / linux-job	4m 22s
test-native-any / linux-job	7m 9s
test-native-cpu / linux-job	7m 10s
	3h 8m 7s

Provide feedback