Add `LlamaBackbone` #3873

Job	Run time
Check the code format	1m 49s
Test the code with tf.keras	24m 35s
Test the code with Keras Core (tensorflow)	24m 32s
Test the code with Keras Core (jax)	29m 11s
Test the code with Keras Core (torch)	15m 54s
	1h 36m 1s

Provide feedback