Rename .cpp file to .cu for py_itfs_cu

[xw285cornell]

The files under py_itfs_cu all reference ck headers, and therefore they cannot be compiled by the normal clang, which complains the following:

In file included from csrc/py_itfs_cu/asm_mha_varlen_bwd.cpp:8:
In file included from csrc/include/py_itfs_common.h:7:
In file included from csrc/include/aiter_hip_common.h:8:
In file included from composable_kernel/include/ck_tile/core.hpp:6:
In file included from composable_kernel/include/ck_tile/core/algorithm/cluster_descriptor.hpp:7:
In file included from composable_kernel/include/ck_tile/core/algorithm/coordinate_transform.hpp:7:
In file included from composable_kernel/include/ck_tile/core/container/multi_index.hpp:7:
In file included from composable_kernel/include/ck_tile/core/container/array.hpp:13:
In file included from composable_kernel/include/ck_tile/core/utility/functional.hpp:9:
In file included from composable_kernel/include/ck_tile/core/container/sequence.hpp:9:
composable_kernel/include/ck_tile/core/numeric/math.hpp:173:28: error: redefinition of 'max'
  173 | CK_TILE_DEVICE constexpr T max(T x, T y)
      |                            ^
composable_kernel/include/ck_tile/core/numeric/math.hpp:167:26: note: previous definition is here
  167 | CK_TILE_HOST constexpr T max(T x, T y)
      |                          ^

Our build rules uses hipclang to compile .cu files, and normal clang for .cpp files. So wondering if we can make this change to make our tools easier.

[valarLip]

looks ok for us

[xw285cornell]

@valarLip can we merge it if it looks ok to you? :)

[valarLip]

yeah.. but we have wait CI finished

[xw285cornell]

For sure!