Add C++ unit tests for cadence::fully_connected.out (#19165)

[hsharma35]

Summary:

Adds C++ unit tests for cadence::fully_connected.out in test_op_fully_connected.cpp. Tests verify correct dense layer output (y = Wx + b) across various input sizes, output feature counts, and with and without bias.

Reviewed By: zonglinpeng, mcremon-meta

Differential Revision: D97889888

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/19165

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❌ 3 Cancelled Jobs, 4 Pending, 2 Unrelated Failures

As of commit 3f3d684 with merge base 9207001 ():

CANCELLED JOBS - The following jobs were cancelled. Please retry:

• pull / test-lora-linux / linux-job (gh)
  ##[error]The operation was canceled.
• pull / test-lora-multimethod-linux / linux-job (gh)
  ##[error]The operation was canceled.
• pull / test-phi-3-mini-runner-linux / linux-job (gh)
  ##[error]The operation was canceled.

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

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

• pull / test-models-linux (phi_4_mini, portable, linux.4xlarge.memory) / linux-job (gh) (trunk failure)
  ##[error]The operation was canceled.
• pull / test-voxtral-realtime-xnnpack-linux / linux-job (gh) (trunk failure)
  ##[error]The operation was canceled.

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[meta-codesync]

@hsharma35 has exported this pull request. If you are a Meta employee, you can view the originating Diff in D97889888.

[meta-codesync]

@hsharma35 has exported this pull request. If you are a Meta employee, you can view the originating Diff in D97889888.

[github-actions]

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