portable: Add const to local vars to suppress CPPCHECK warning

[vacu9708]

e93a285ebd "Extend CPPCHECK scope to portable kernels"
My work branch was cut before this new lint coverage.

Summary

CPPCHECK flagged apply_unary_map_reduce_fn<CTYPE, ACC> with a constStatement warning. This is a false positive — the code compiles correctly and the result is assigned to a variable used in subsequent computation. Adding const to the declarations suppresses the warning.

Test

$ lintrunner op_softmax.cpp op_log_softmax.cpp op_mean.cpp op_sum.cpp \
             op_softmax_test.cpp op_log_softmax_test.cpp op_mean_test.cpp op_sum_test.cpp
ok  No lint issues.

cc @digantdesai @freddan80 @per @zingo @oscarandersson8218 @mansnils @Sebastian-Larsson @robell @rascani

[pytorch-bot]

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❌ 6 New Failures, 1 Cancelled Job, 3 Unrelated Failures, 1 Unclassified Failure

As of commit 39aa25c with merge base 574bfca ():

NEW FAILURES - The following jobs have failed:

• pull / test-arm-backend-no-driver (test_pytest_models_tosa) / linux-job (gh)
  RuntimeError: Command docker exec -t c400f7c1a30d819203c1464e6f469acc39838a9e9a3eec2eace455a116288cac /exec failed with exit code 1
• pull / test-phi-3-mini-runner-linux / linux-job (gh)
  RuntimeError: Command docker exec -t f4d27d45bf837211cc370dafac9a507b34f88785677b8ff385b843c64e0e473a /exec failed with exit code 1
• trunk / test-arm-backend-ethos-u (test_pytest_models_ethos_u55) / linux-job (gh)
  RuntimeError: Command docker exec -t ea51b3c70e10423528d0b93fdd0f3ff2c8e74655dc72b07f6724d637a0af7ae1 /exec failed with exit code 1
• trunk / test-arm-backend-ethos-u (test_pytest_models_ethos_u85) / linux-job (gh)
  RuntimeError: Command docker exec -t b293571837fce053dafb81d18c26be3dce98aeadb896c94e574d04e2e4775c9c /exec failed with exit code 1
• trunk / test-arm-backend-vkml (test_pytest_models_vkml) / linux-job (gh)
  RuntimeError: Command docker exec -t 3efd17536ca84893fec2e002d76690f2df346e721a871a31aa22d15de7d18dd8 /exec failed with exit code 1
• trunk / unittest-release / macos / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1

UNCLASSIFIED FAILURE - DrCI could not classify the following job because the workflow did not run on the merge base. The failure may be pre-existing on trunk or introduced by this PR:

• trunk / test-models-macos-coreml (mv3) / macos-job (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1

CANCELLED JOB - The following job was cancelled. Please retry:

• pull / test-moshi-linux / linux-job (gh)
  ##[error]The operation was canceled.

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• pull / test-qnn-testsuite-linux / test-backend-linux (qnn, models) / linux-job (gh) (detected as infra flaky with no log or failing log classifier)

BROKEN TRUNK - The following jobs failed but was present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / test-llama-runner-qnn-linux (fp32, qnn_16a16w, qnn) / linux-job (gh) (trunk failure)
• trunk / test-llama-runner-qnn-linux (fp32, qnn_16a16w, qnn) / linux-job (gh) (trunk failure)

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[Gasoonjia]

LGTM. Since it touched arm code space more than lint, I will defer to experts @digantdesai @SS-JIA for stamp.

[vacu9708]

@Gasoonjia
Oh I am sorry by mistake I included files I am currently working on for the arm backend in this commit.
I excluded all the ARM-related changes.