feat: add `GGML_UNARY_OP_ARGMAX` Metal kernel (#1019)

feat: add `GGML_UNARY_OP_ARGMAX` Metal kernel (#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 <redacted>
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Co-authored-by: Diego Devesa <redacted>