feat: add `GGML_UNARY_OP_ARGMAX` Metal kernel (ggml/1019)
* implemented argmax kernel
* tpig -> tgpig
* change to strides
* contiguous assertions
* kernel working and tested
* argmax simd parallel implementation
* added 2 new tests for argmax in test-backend-ops
* cosmit
* added 3 tests cases for perf eval
* add test_argmax in make_test_cases_perf
* Update test-backend-ops.cpp
Co-authored-by: Diego Devesa <slarengh@gmail.com>
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Co-authored-by: Diego Devesa <slarengh@gmail.com>
