Vulkan: add the missing int32 -> uint8 view_convert variant

[msluszniak]

Summary

Fixes #22320.

The view_convert combo lists generate uint8 -> int32 but not the reverse, so a graph containing an int32 -> uint8 view_convert lowers cleanly through VulkanPartitioner and then aborts on device:

Could not find ShaderInfo with name view_convert_buffer_int32_uint8
(ShaderRegistry.cpp:54, (it != listings_.end()) is false)

The partitioner has no visibility into which dtype pairs were generated, so the node is accepted at AOT and the failure only appears at run time. sentence-transformers/all-MiniLM-L6-v2 hits this through its attention mask on any graph lowered with VulkanPartitioner.

view_convert is a plain elementwise OUT_T(t_inp[...]) cast, so the pair needs nothing beyond being listed in the buffer and texture variant lists.

Test plan

On a Galaxy S26 Ultra (Adreno 840), arm64-v8a, NDK 27.1, main @ c27baa8:

Before, executor_runner aborts loading the model with the error above. After, all-MiniLM-L6-v2 loads and runs, and its embedding matches the eager reference:

minilm  vulkan vs torch ref   max|dev| 4.0e-04   cosine 0.99999726
minilm  replay 0 vs replay 29 max|dev| 0.0       cosine 1.00000000

Bit-identical across 90 in-process replays (3 rounds x 30 executions), so the added cast is stable as well as correct. Median 7.76 ms/iteration against 8.38 ms for the same model on XNNPACK.

cc @SS-JIA @manuelcandales @digantdesai @cbilgin

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⚠️ 15 Awaiting Approval

As of commit add7363 with merge base c27baa8 ():

AWAITING APPROVAL - The following workflows need approval before CI can run:

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