[ET-VK] Replace dynamic UBO indexing with safe_idx across shaders

[SS-JIA]

Stack from ghstack (oldest at bottom):

• -> [ET-VK] Replace dynamic UBO indexing with safe_idx across shaders #18512
• [ET-VK] Modernize permute op with safe indexing and unified dispatch #18511

The Adreno 740 GPU driver crashes (SIGSEGV in vkCreateComputePipelines)
when GLSL shaders dynamically index a UBO-backed ivec4/ivec3 with a
specialization-constant-derived value. This was causing the skin
segmentation model to crash during pipeline creation on Samsung S23.

Fix all instances across 18 shader files by replacing patterns like
meta.sizes[packed_dim] with safe_idx(meta.sizes, packed_dim), which
uses an if/else chain that the driver resolves at pipeline creation time.

Changes:

• Add safe_idx(ivec3) overload to indexing.glslh
• Fix transfer_texture.glsl, slice.glslh, select.glslh (transfer ops)
• Fix nchw_to_int8x4_buffer.glsl, full_texture.glsl (staging/utility)
• Fix gather, split, index_tensor, where, expand, pad, repeat, arange
  texture shaders (1-line fixes each)
• Fix softmax.glsl, reduce.glsl, reduce2d.glsl, var_texture3d.glsl
  (reduction shaders with multiple fixes + added indexing.glslh include)
• Remove unused ShaderNameUtils.h include from Slice.cpp

Differential Revision: D98220450

[pytorch-bot]

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