[compiler][python] Make target_backends optional #11472

Job	Run time
setup / setup	12s
Build Packages / Linux Release (x86_64)	12m 14s
Test AMD MI300	0s
Test AMD W7900	0s
Test AMD MI250	0s
Test NVIDIA T4	0s
Regression Test / test_onnx :: cpu_llvm_sync	3m 11s
Regression Test / test_onnx :: amdgpu_rocm_rdna3	7m 8s
Regression Test / test_onnx :: amdgpu_vulkan	1m 50s
Regression Test / test_models :: cpu_llvm_task	5m 38s
Regression Test / test_regression_suite :: cpu_llvm_task	10m 49s
Regression Test / test_models :: amdgpu_rocm_mi250_gfx90a	4m 13s
Regression Test / test_onnx :: nvidiagpu_cuda	5m 46s
Regression Test / test_regression_suite :: amdgpu_rocm_mi250_gfx90a	4m 22s
Regression Test / test_models :: amdgpu_rocm_mi300_gfx942	4m 25s
Regression Test / test_onnx :: nvidiagpu_vulkan	4m 58s
Regression Test / test_regression_suite :: amdgpu_rocm_mi300_gfx942	6m 40s
Regression Test / test_models :: amdgpu_vulkan	2m 21s
Unit Test / Linux (x86_64)	26s
test_android / cross_compile	5m 12s
Test TensorFlow CPU / Linux (x86_64)	2m 14s
	1h 21m 39s

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