PyYAML is not an actual use-time/runtime dependency of our bindings. It
is necessary only if someone wants to regenerate
`LinalgNamedStructuredOps.yaml`:
https://github.com/llvm/llvm-project/blob/93097b2d47c87bf5eee0a2612d961c7a01831eab/mlir/tools/mlir-linalg-ods-gen/update_core_linalg_named_ops.sh.in#L29

This PR does the minimal refactor to remove the need during actual run/use time.

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

Fixes https://github.com/llvm/llvm-project/issues/168393. Also adds
top-level `MLIR_PYTHON_STUBGEN_ENABLED` CMake option.

I missed these attributes when I added the wrapper for GPUFuncOp in
fbdd98f74f0d.

Makes it so that a NamedSequenceOp can be directly applied to a Module,
via a method `apply(...)`.

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.

Makes linalg.reduce and linalg.map region_ops so they can be constructed
from functions and be called as decorators.

The C++ index switch op has utilities for `getCaseBlock(int i)` and
`getDefaultBlock()`, so these have been added.
Optional body builder args have been added: one for the default case and
one for the switch cases.

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