Don't force extension_module to build as a shared library by default

[swolchok]

Other targets seem to just use whatever the default configuration is. Make this one work the same.

[swolchok]

Stack from ghstack (oldest at bottom):

• -> Don't force extension_module to build as a shared library by default #12257

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/12257

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❌ 4 New Failures, 2 Unrelated Failures

As of commit c9c0aed with merge base d952326 ():

NEW FAILURES - The following jobs have failed:

• pull / test-llama-runner-qnn-linux (fp32, qnn_16a16w, qnn) / linux-job (gh)
  undefined reference to executorch::runtime::TensorLayout::create(executorch::runtime::Span, executorch::runtime::Span, executorch::runtime::etensor::ScalarType)'`
• pull / test-llama-runner-qnn-linux (fp32, qnn_8a8w, qnn) / linux-job (gh)
  undefined reference to executorch::runtime::TensorLayout::create(executorch::runtime::Span, executorch::runtime::Span, executorch::runtime::etensor::ScalarType)'`
• pull / test-llava-runner-linux / linux-job (gh)
  undefined reference to executorch::extension::MmapDataLoader::~MmapDataLoader()'`
• pull / test-static-llama-qnn-linux / linux-job (gh)
  undefined reference to executorch::runtime::TensorLayout::create(executorch::runtime::Span, executorch::runtime::Span, executorch::runtime::etensor::ScalarType)'`

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• pull / test-models-linux (mobilebert, portable, linux.2xlarge) / linux-job (gh) (similar failure)
  ##[error]The operation was canceled.

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

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

• pull / test-models-linux (mobilebert, xnnpack-quantization-delegation, linux.2xlarge) / linux-job (gh) (trunk failure)
  ##[error]The operation was canceled.

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[swolchok]

hmm. the llava-runner build failure doesn't repro on Mac and I'm currently having internal-specific trouble building on my Linux machine.