Cortex-M backend: Fix conv2d scratch buffer allocation to match CMSIS-NN wrapper dispatch

[rascani]

Summary

Use arm_convolve_wrapper_s8_get_buffer_size instead of arm_convolve_s8_get_buffer_size so the buffer size matches whichever specialized kernel arm_convolve_wrapper_s8 will actually dispatch to at runtime (1x1 fast, 1xN, or general).

Also remove the Error::NotFound carve-out that silently proceeded with a null scratch buffer — CMSIS-NN returns ARM_CMSIS_NN_ARG_ERROR when ctx->buf is NULL and a buffer is required, so fail immediately on any allocation error, consistent with the other cortex_m conv ops.

Update CMSIS-NN from v7.0.0 to 84303a51fd867c7ddbd23068b7ce930af1b6269d
and remove GIT_SHALLOW (incompatible with SHA-based FetchContent pins).

Fixes #18044

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

[pytorch-bot]

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❌ 1 New Failure, 1 Pending, 1 Unrelated Failure

As of commit 94cbb57 with merge base 6c02866 ():

NEW FAILURE - The following job has failed:

• pull / unittest / macos / macos-job (gh)
  export/tests/test_target_recipes.py::TestTargetRecipes::test_linear_model

BROKEN TRUNK - The following job failed but were present on the merge base:

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

• pull / test-binary-size-linux-gcc / linux-job (gh) (trunk failure)

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[rascani]

Merge will be blocked until ARM-software/CMSIS-NN#200 has merged and we can update the CMSIS version.

[rascani]

Failures unrelated.