Cortex-M backend: Add quantized int8 batch matmul (CMSIS-NN)

[rascani]

Summary

Add cortex_m::quantized_batch_matmul wrapping arm_batch_matmul_s8. The RHS is always pre-transposed: constant RHS (parameters) are transposed at AOT time in the pass, dynamic RHS get a cortex_m::transpose node inserted in the graph.

It would be preferable if we could pre-compute or cache the constant RHS kernel sums, but I could not find any public CMSIS-NN APIs that would allow us to do so.

Fixes #16109

Authored with Claude.

Test plan

pytest backends/cortex_m/test/ops/test_batch_matmul.py

[pytorch-bot]

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❌ 1 Awaiting Approval, 3 New Failures

As of commit cb40758 with merge base 25f2a3f ():

AWAITING APPROVAL - The following workflow needs approval before CI can run:

• periodic (gh)

NEW FAILURES - The following jobs have failed:

• Build Presets / linux (llm, linux.2xlarge, executorch-ubuntu-22.04-clang12) / build (gh)
• pull / test-samsung-models-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 468eaf869ee411154b4e2fdc2282e32d0e5d85d49fe6045cb770b7f15d0f16d6 /exec failed with exit code 1
• pull / test-samsung-quantmodels-linux / linux-job (gh)
  RuntimeError: Command docker exec -t e6f72e580c2392368c765f927d59fdfe31bf390b763d9dabf95b2a7cb7245c6b /exec failed with exit code 1

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