portable: add fast path for contiguous innermost-dim reduction in amax.out and amin.out

[vacu9708]

Motivation

sum.IntList_out, mean.out, and var_mean.correction_out special-case the contiguous, single innermost-dim reduction shape with a direct pointer loop instead of the generic ReduceOverDimListPlan, which recomputes a stride-based starting index per output element via get_init_index (#18789, #18790, #18791). amax.out and amin.out do not have this fast path.

Tests

No new tests were added. The existing tests already cover both fast and generic paths.
sum's (#18789), mean's (#18790), and var_mean's (#18791) fast-path PRs had no test changes either.

Test	Shape	dim	Fast path?	Dtypes covered
AllRealInputOutputPasses	{2,3,4}	{2} (innermost)	Yes	ET_FORALL_REALHBBF16_TYPES (incl. Bool, Half, BFloat16)
AllRealInputOutputPasses	{2,3,4}	{-2} (not innermost)	No, fallback	same
AllRealInputOutputPasses	{2,3,4}	{0,1} (multi-dim)	No, fallback	same
AllRealInputOutputPasses	{2,2,4}	null / empty	No, fallback	same
InfinityAndNANTest	{2,3,4}	{-1} (innermost)	Yes	ET_FORALL_FLOATHBF16_TYPES

Same coverage in both op_amax_test.cpp and op_amin_test.cpp.

$ ninja -C cmake-out-bench portable_kernels_test -j16
[6/6] Linking CXX executable kernels/test/portable_kernels_test

$ ./cmake-out-bench/kernels/test/portable_kernels_test
[==========] Running 1625 tests from 190 test suites.
[  PASSED  ] 1521 tests.
# 0 FAILED, 209 SKIPPED (pre-existing DISABLED/Aten-only tests, unrelated to this change)

$ ./cmake-out-bench/kernels/test/portable_kernels_test --gtest_filter="OpAmaxOutTest.*:OpAminOutTest.*"
[==========] 10 tests from 2 test suites ran.
[  PASSED  ] 10 tests.

$ lintrunner -a kernels/portable/cpu/op_amax.cpp kernels/portable/cpu/op_amin.cpp
ok No lint issues.

[pytorch-bot]

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