[executorch][nvidia][tensorrt][2/n] Add TensorRT backend stub

[shoumikhin]

Stack from ghstack (oldest at bottom):

• [executorch][nvidia][tensorrt][25/n] Share CUDA stream across TRT delegates #17936
• [executorch][nvidia][tensorrt][24/n] Add power and unary converters with model improvements #17935
• [executorch][nvidia][tensorrt][23/n] Add benchmark infrastructure #17934
• [executorch][nvidia][tensorrt][22/n] Add correctness tests #17933
• [executorch][nvidia][tensorrt][21/n] Add comparison and slice converters #17932
• [executorch][nvidia][tensorrt][20/n] Add bmm converter #17931
• [executorch][nvidia][tensorrt][19/n] Add SDPA converter #17930
• [executorch][nvidia][tensorrt][18/n] Add layer_norm and pixel_shuffle converters #17929
• [executorch][nvidia][tensorrt][17/n] Add embedding, expand, upsample converters #17928
• [executorch][nvidia][tensorrt][16/n] Enable MobileNetV3 model support #17927
• [executorch][nvidia][tensorrt][15/n] Add linear model support with addmm and permute_copy converters #17926
• [executorch][nvidia][tensorrt][14/n] Add additional converters (mul, sub, div, mm, relu) #17925
• [executorch][nvidia][tensorrt][13/n] Add examples, C++ runner and CI workflow #17924
• [executorch][nvidia][tensorrt][12/n] Add C++ runtime infrastructure #17923
• [executorch][nvidia][tensorrt][11/n] Complete preprocess integration with serialization #17922
• [executorch][nvidia][tensorrt][10/n] Add blob serialization format with I/O metadata #17921
• [executorch][nvidia][tensorrt][9/n] Implement preprocess basic implementation #17920
• [executorch][nvidia][tensorrt][8/n] Add TensorRTCompileSpec dataclass #17919
• [executorch][nvidia][tensorrt][7/n] Add converter utils and add converter #17918
• [executorch][nvidia][tensorrt][6/n] Add converter registry infrastructure #17917
• [executorch][nvidia][tensorrt][5/n] Integrate partitioner #17916
• [executorch][nvidia][tensorrt][4/n] Add operator support base class #17915
• [executorch][nvidia][tensorrt][3/n] Add TensorRT partitioner stub #17914
• -> [executorch][nvidia][tensorrt][2/n] Add TensorRT backend stub #17913
• [executorch][nvidia][tensorrt][1/n] Add directory structure and initialization #17912

Add initial backend stub with empty preprocess implementation and TensorRT backend registration.

Differential Revision: D93275053

[pytorch-bot]

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🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/17913

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❌ 15 New Failures, 2 Unrelated Failures

As of commit fc6f16f with merge base 01d21fa ():

NEW FAILURES - The following jobs have failed:

• Test CUDA Builds / check-all-cuda-builds (gh)
  Process completed with exit code 1.
• Test CUDA Builds / export-model-cuda-artifact (nvidia, parakeet-tdt, non-quantized) / linux-job (gh)
  RuntimeError: Command docker exec -t 88cba2a19c44413ddc9994d98a874714aeeeeca71ea0e5aae8b3fab1344fe90d /exec failed with exit code 1
• Test CUDA Builds / export-model-cuda-artifact (openai, whisper-large-v3-turbo, quantized-int4-tile-packed) / linux-job (gh)
  RuntimeError: Command docker exec -t fdf6a074b77a6634f1be397f238014f956264c7d747e244b06311223172a1615 /exec failed with exit code 1
• Test CUDA Builds / export-model-cuda-artifact (Qwen, Qwen3-0.6B, quantized-int4-tile-packed) / linux-job (gh)
  RuntimeError: Command docker exec -t ac36610167b19ae9a6317115e3828804809e890b8fb66bd7dfbfb8d1bd096a44 /exec failed with exit code 1
• Test CUDA Builds / test-executorch-cuda-build-12.8 / linux-job (gh)
  RuntimeError: Command docker exec -t edd3206bcb33dc19fccf252c49dcdc4efc0c76b2dea9ac2c9d7f77382abbcbe7 /exec failed with exit code 1
• Test CUDA Builds / test-models-cuda (linear) / linux-job (gh)
  RuntimeError: Command docker exec -t 798b6c91c1c560faf651db5b0c121f4db61b7a1493512e661af68fe7bb2a2389 /exec failed with exit code 1
• Test CUDA Builds / test-models-cuda (resnet18) / linux-job (gh)
  RuntimeError: Command docker exec -t 3568d2ae5a9837e4440c9cdd6fce71301e4d1b67ec6086ffba4e93b309b656c2 /exec failed with exit code 134
• Test CUDA Windows Export and E2E / export-model-cuda-windows-artifact (mistralai, Voxtral-Mini-3B-2507, quantized-int4-weight-only) / linux-job (gh)
  RuntimeError: Command docker exec -t c4456b085d0ab5b5581ac2de162f696766ac738d6b1a338c2d5e16a239efe735 /exec failed with exit code 1
• Test Metal Backend / export-model-metal-artifact (mistralai, Voxtral-Mini-3B-2507, non-quantized) / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1
• Test Metal Backend / export-model-metal-artifact (mistralai, Voxtral-Mini-3B-2507, quantized-int4-metal) / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1
• Test Metal Backend / export-model-metal-artifact (mistralai, Voxtral-Mini-4B-Realtime-2602, quantized-int4-metal) / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1
• Test Metal Backend / export-model-metal-artifact (nvidia, parakeet-tdt, non-quantized) / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1
• Test Metal Backend / export-model-metal-artifact (nvidia, parakeet-tdt, quantized-int4-metal) / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1
• Test Metal Backend / export-model-metal-artifact (openai, whisper-large-v3-turbo, non-quantized) / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1
• Test Metal Backend / export-model-metal-artifact (openai, whisper-small, non-quantized) / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1

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

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

• pull / unittest / macos / macos-job (gh) (trunk failure)
  AttributeError: '_OpNamespace' 'mkldnn' object has no attribute '_is_mkldnn_acl_supported'
• pull / unittest-editable / macos / macos-job (gh) (trunk failure)
  AttributeError: '_OpNamespace' 'mkldnn' object has no attribute '_is_mkldnn_acl_supported'

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