[ROCm] Set thread work size to 8 for elementwise kernels

[jerrymannil]

This improves elementwise kernel performances on MI300 gpus.

Co-author: @akadutta

[jerrymannil]

test_unary_unfunc, test_binary_ufunc are passing
test_transformers also is passing, except for 6 preexisting failures.

[rocm-mici]

Jenkins build for d33205785a2d6458032045206b0ab45dc932729b commit finished as FAILURE
Links: Blue Ocean view / Build artifacts

Detected error during Pytorch building:

  580 | #define DEPRECATED(msg) __attribute__ ((deprecated(msg)))
      |                                         ^
1 warning generated when compiling for gfx908.
[7982/8635] Building HIPCC object caffe2/CMakeFiles/torch_hip.dir/__/aten/src/ATen/native/hip/torch_hip_generated_spherical_bessel_j0.hip.o
[7983/8635] Building HIPCC object caffe2/CMakeFiles/torch_hip.dir/__/aten/src/ATen/native/transformers/hip/flash_attn/torch_hip_generated_flash_api.hip.o
FAILED: caffe2/CMakeFiles/torch_hip.dir/__/aten/src/ATen/native/transformers/hip/flash_attn/torch_hip_generated_flash_api.hip.o /var/lib/jenkins/pytorch/build/caffe2/CMakeFiles/torch_hip.dir/__/aten/src/ATen/native/transformers/hip/flash_attn/torch_hip_generated_flash_api.hip.o 
cd /var/lib/jenkins/pytorch/build/caffe2/CMakeFiles/torch_hip.dir/__/aten/src/ATen/native/transformers/hip/flash_attn && /opt/conda/envs/py_3.10/bin/cmake -E make_directory /var/lib/jenkins/pytorch/build/caffe2/CMakeFiles/torch_hip.dir/__/aten/src/ATen/native/transformers/hip/flash_attn/. && /opt/conda/envs/py_3.10/bin/cmake -D verbose:BOOL=OFF -D build_configuration:STRING=RELEASE -D generated_file:STRING=/var/lib/jenkins/pytorch/build/caffe2/CMakeFiles/torch_hip.dir/__/aten/src/ATen/native/transformers/hip/flash_attn/./torch_hip_generated_flash_api.hip.o -P /var/lib/jenkins/pytorch/build/caffe2/CMakeFiles/torch_hip.dir/__/aten/src/ATen/native/transformers/hip/flash_attn/torch_hip_generated_flash_api.hip.o.cmake
In file included from /var/lib/jenkins/pytorch/aten/src/ATen/native/transformers/hip/flash_attn/flash_api.hip:57:
/var/lib/jenkins/pytorch/aten/src/ATen/native/transformers/hip/aotriton_adapter.h:120:10: error: no matching constructor for initialization of 'aotriton::TensorView<0>'
  120 |   return aotriton::TensorView<0>(reinterpret_cast<intptr_t>(q.data_ptr()),
      |          ^                       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~