Fix MHC import: don't import .torch from layers/mhc.py

The layers/mhc.py was trying to import kernels.mhc.torch which
failed because our __init__.py was breaking the package. Instead,
just import our mhc_torch_ops which has everything we need.

Also fix __init__.py to explicitly import mhc_pre_torch and
mhc_post_torch from .torch instead of using import *.

vllm/patches/layers/mhc.py

@@ -8,8 +8,6 @@ from vllm.model_executor.custom_op import CustomOp
 
 # Import our torch implementations (registers torch.ops.vllm.mhc_pre, etc.)
 import vllm.model_executor.kernels.mhc.mhc_torch_ops as _mhc_torch  # noqa: F401
-# Also import the original torch impls (mhc_pre_torch, mhc_post_torch)
-import vllm.model_executor.kernels.mhc.torch as mhc_kernels  # noqa: F401
 
 
 @CustomOp.register("mhc_pre")