[compiler][python] Make target_backends optional (#18151) #2671

Job	Run time
setup / setup	5s
build_for_benchmarks / build_all	7m 49s
build_e2e_test_artifacts / build_e2e_test_artifacts	15m 37s
build_benchmark_tools / build_benchmark_tools (linux, x86_64, gcr.io/iree-oss/base-bleeding-edge@sha256:cf2e78194e64fd016...	2m 3s
build_benchmark_tools / build_benchmark_tools (linux, riscv_64, gcr.io/iree-oss/riscv@sha256:62e87bad3405d691ddba6f9be0ef...	2m 56s
build_benchmark_tools / build_benchmark_tools (android, armv8.2-a, gcr.io/iree-oss/android@sha256:66b92a1c920588a73d03316...	2m 6s
test_benchmark_suites (linux, riscv_64, gcr.io/iree-oss/riscv@sha256:62e87bad3405d691ddba6f9be0ef...	3m 17s
test_benchmark_suites (linux, riscv_32, gcr.io/iree-oss/riscv@sha256:62e87bad3405d691ddba6f9be0ef...	3m 3s
test_benchmark_suites (linux, x86_64, gcr.io/iree-oss/base@sha256:dc314b4fe30fc1315742512891357bf...	2m 33s
execution_benchmarks / generate_matrix	16s
compilation_benchmarks / compilation_benchmarks	40s
execution_benchmarks / run_benchmarks (c2-standard-60, linux, x86_64, 0, 2)	10m 6s
execution_benchmarks / run_benchmarks (c2-standard-60, linux, x86_64, 1, 2)	9m 52s
process_benchmark_results	3m 18s
benchmark_summary	4s
	1h 3m 45s

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