Optimize index_out via fast path

[manuelcandales]

Differential Revision: D81142086

[pytorch-bot]

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• Multiple CI trunk failures after landing https://github.com/pytorch/pytorch/pull/161002

❌ 1 New Failure, 1 Cancelled Job

As of commit 244302f with merge base 4c4cbdf ():

NEW FAILURE - The following job has failed:

• pull / test-binary-size-linux-gcc / linux-job (gh)
  /pytorch/executorch/kernels/portable/cpu/op_index.cpp:112:11: error: comparison of integer expressions of different signedness: ‘const long int’ and ‘size_t’ {aka ‘long unsigned int’} [-Werror=sign-compare]

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

• periodic (gh)

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This pull request was exported from Phabricator. Differential Revision: D81142086

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