[ET-VK][q8_ops] Add int8x4_buffer_to_nchw shader and refactor Int8x4Staging

[SS-JIA]

Stack from ghstack (oldest at bottom):

• [ET-VK] Add aten.index.Tensor op and tests #17710
• [ET-VK] Add Vulkan ops for skin segmentation and EdgeTAM models #17709
• -> [ET-VK][q8_ops] Add int8x4_buffer_to_nchw shader and refactor Int8x4Staging #17708
• [ET-VK][ez] Fix global workgroup size underflow for sub-4D tensors in block config dispatch #17707

Renames Q8taStaging.cpp/h to Int8x4Staging.cpp/h and expands it to cover
the full staging lifecycle for kInt8x4 buffer tensors.

Rename and split of the old prepack function:
The old add_staging_to_int8x4_buffer_node (which used a static dispatch
node for prepacking TensorRef data into a packed int8x4 buffer) is renamed
to add_prepack_int8x4_buffer_node to clarify its role. Two new runtime
staging functions are added alongside it:

• add_staging_to_int8x4_buffer_node: reads NCHW data from a staging buffer
  into a kInt8x4 buffer tensor at execute time, using a DynamicDispatchNode
  wrapping the existing nchw_to_int8x4_buffer shader.
• add_int8x4_buffer_to_staging_node: writes packed int8x4 data back from a
  kInt8x4 buffer tensor to a contiguous NCHW staging buffer at execute time,
  using a new int8x4_buffer_to_nchw shader.

New shader (int8x4_buffer_to_nchw.glsl):
Implements the reverse of nchw_to_int8x4_buffer. One thread per output
int32 in the NCHW staging buffer. For each thread it decodes 4 NCHW-ordered
element indices, looks up each element's position in the packed int8x4 buffer
via tensor4d_idx_to_buf_idx, extracts the packed byte, and assembles 4
bytes into a single output int32. Works for any GPUMemoryLayout.

Staging.cpp dispatch:
add_staging_to_tensor_node and add_tensor_to_staging_node now both
dispatch to the int8x4-specific functions when the tensor dtype is kInt8x4.
prepack_op is updated to call add_prepack_int8x4_buffer_node.

TestQ8taBinary.cpp is updated to include Int8x4Staging.h and call
add_prepack_int8x4_buffer_node.

Differential Revision: D94364640

[pytorch-bot]

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❌ 1 New Failure

As of commit 1c319ca with merge base 63f9724 ():

NEW FAILURE - The following job has failed:

• Test CUDA Builds / test-model-cuda-e2e (mistralai, Voxtral-Mini-3B-2507, non-quantized) / linux-job (gh)
  RuntimeError: Command docker exec -t 3f7d03ebf505b37be559498254c36ec2c7b6e9708380af2b04bf01372652a035 /exec failed with exit code 55

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