Layernorm welford (#346)

* Add threadwise and blockwise welford

* Rename gridwise op, prepare to add welford version

* implement welford and integrate welford into layernorm

* Take care of tail loop

* Fix buf when ThreadSliceK > 1

* Fix bug of merging of two empty set

* Rename clip to clamp

* 1. Fix type of count
2. Remove useless static_assert

* Do not inherit Reduction::Argument

* [What] replace __syncthreads() with block_sync_lds()
[Why] __syncthreads might wait both lgkmcnt(0) and vmcnt(0)

* Add y stride

* Rename.
DeviceLayernorm -> DeviceLayernormImpl
DeviceNormalization2 -> DeviceLayernorm

* Move literal ""_uz & ""_zu into namespace 'literals'

* Move namespace 'literals' as 'ck::literals'

Co-authored-by: Po-Yen, Chen <PoYen.Chen@amd.com>
Co-authored-by: Chao Liu <chao.liu2@amd.com>

include/ck/utility/math.hpp

@@ -144,6 +144,12 @@ __host__ __device__ constexpr auto min(X x, Ys... ys)
     return min(x, min(ys...));
 }
 
+template <typename T>
+__host__ __device__ constexpr T clamp(const T& x, const T& lowerbound, const T& upperbound)
+{
+    return min(max(x, lowerbound), upperbound);
+}
+
 // disallow implicit type casting
 template <typename T>
 __device__ T exp(T x);