[Ck tile] Use raw store to improve layernorm performance

[rocking5566]

Proposed changes

1. Simpler kernel example for layernorm
2. use store_tile_raw for Default2DEpilogueProblem to improve performance

Checklist

use following command to check performance
make -j tile_layernorm2d_fwd && ./bin/tile_layernorm2d_fwd -m=128 -n=8192 -prec_i=bf16 -fadd=1

[spolifroni-amd]

Nothing really to review in terms of docs. Let me know if anything needs to be documented.