Turn on layer scanning for llama2-7b on GPU.
Turn on layer scanning for llama2-7b on GPU.

This better utilizes recent optimizations to collective approximation in the XLA latency hiding scheduler.

FUTURE_COPYBARA_INTEGRATE_REVIEW=https://github.com/openxla/xla/pull/13639 from philipphack:u_layer_uaf_xla 91ebf7b4a2ac90ebadce27d1a73e88fb4513aed4
