Fix out-of-bounds write in Cadence HiFi bmm bias buffer initialization

[rascani]

Summary

The temporary zero-bias buffer for xa_nn_matmul_f32xf32_f32 was being "initialized" by writing a single zero one element past the end of the allocation (tmp[batch_size * m * p] = {0}). Replace with memset to correctly zero the entire buffer, and use sizeof(FLOAT32) consistently.

This diff was authored with the help of Claude.

Test plan

CI

[pytorch-bot]

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❌ 3 New Failures, 3 Unrelated Failures

As of commit b567cd6 with merge base c12dc35 ():

NEW FAILURES - The following jobs have failed:

• pull / test-samsung-models-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 35c409ab6327ae6223602e6f72545cf1ed6be41618839feb89948d86793907b6 /exec failed with exit code 1
• pull / test-samsung-quantmodels-linux / linux-job (gh)
  RuntimeError: Command docker exec -t ea0338756d2ec07cb3d14bb9f9024e253f2f518e6e2c7f1af3d9c9d9ce20824f /exec failed with exit code 1
• pull / unittest-editable / macos / macos-job (gh)
  export/tests/test_target_recipes.py::TestTargetRecipes::test_mv3_model

FLAKY - The following jobs failed but were likely due to flakiness present on trunk:

• Test CUDA Builds / test-models-cuda (mv2) / linux-job (gh) (detected as infra flaky with no log or failing log classifier)
• Test CUDA Builds / test-models-cuda (resnet18) / linux-job (gh) (detected as infra flaky with no log or failing log classifier)
• Test CUDA Builds / test-models-cuda (sdpa) / linux-job (gh) (detected as infra flaky with no log or failing log classifier)

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[mcremon-meta]

cc @dijopaul and @cad-audio

[meta-codesync]

@rascani has imported this pull request. If you are a Meta employee, you can view this in D93289191.

[rascani]

Failures unrelated.