`set_op_layout_attr` and `set_desc_layout` transform ops wrap
`xegpu.layout` in an `xegpu.slice` attribute if `slice_dims` argument is
set.

Adds `transform.xegpu.convert_layout` transform op that inserts an
`xegpu.convert_layout` op for a given `Value`.

Adds `transform.xegpu.insert_prefetch` transform op that inserts
`xegpu.prefetch_nd` ops for the given `Value` in an `scf.for` loop.

Adds `transform.xegpu.set_gpu_launch_threads` that overrides `gpu.launch` operation threads.

Adds `transform.xegpu.set_op_layout_attr` transform op that attaches
`xegpu.layout` attribute to the target op.

Add `transform.xegpu.get_desc_op` transform op that finds a
`xegpu.create_nd_tdesc` producer op of a `Value`.

Adds the first XeGPU transform op, `xegpu.set_desc_layout`, which attachs a `xegpu.layout` attribute to the descriptor that a `xegpu.create_nd_tdesc` op returns.