[ET-VK][qconv][ez] Make q8ta_im2col shader support stride_w != 1

[SS-JIA]

Stack from ghstack (oldest at bottom):

• [ET-VK][profiling] Add additional profiling blocks #17394
• [ET-VK][qconv2d][ez] Don't use im2col path for general convs #17393
• [ET-VK] Add nchw_to_int8x4_buffer shader for prepacking int8 staging data #17392
• [ET-VK] Layout-flexible impl of quantized binary #17391
• [ET-VK][ez] Make q8ta_conv2d use 4C1W layout #17390
• [ET-VK][ez] Add AOT support for PackedInt8_4C1W dtype #17389
• [ET-VK] Migrate to use new q8ta_conv2d ops #17388
• [ET-VK][qconv] Dynamically select between im2col path and general path #17387
• -> [ET-VK][qconv][ez] Make q8ta_im2col shader support stride_w != 1 #17386
• Back out "[Diff Train][pytorch/executorch] Apply fixup patch to fbsource" #17399

The im2col path previously had two shaders: the generic q8ta_im2col
which loaded input elements one int8 at a time and supported all strides,
and q8ta_im2col_4w4c which loaded packed int32s (4 channels at once)
but was restricted to stride_w == 1 because it assumed consecutive
width positions in the input (i.e. input_x_base + i).

This diff replaces both shaders with a single q8ta_im2col that uses
the efficient packed loading approach from q8ta_im2col_4w4c while
generalizing the width offset to input_x_base + i * stride_x. This
removes the stride_w == 1 restriction and deletes the separate
q8ta_im2col_4w4c shader entirely.

The C++ dispatch is updated to reference the unified shader name, and
the test gate on stride.w == 1 for the im2col path is removed.

Authored with Claude.

Differential Revision: D93000161

[pytorch-bot]

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