[AFQ] Optimize tensor_flatten for runtime #4554

Job	Run time
test (CPU 2.2.2, linux.4xlarge, torch==2.2.2 --index-url https://download.pytorch.org/whl/cpu "nu... / linux-job	5m 12s
test (CPU 2.3, linux.4xlarge, torch==2.3.0 --index-url https://download.pytorch.org/whl/cpu, cpu) / linux-job	7m 15s
test (CPU 2.4, linux.4xlarge, torch==2.4.0 --index-url https://download.pytorch.org/whl/cpu, cpu) / linux-job	8m 35s
test (CPU Nightly, linux.4xlarge, --pre torch --index-url https://download.pytorch.org/whl/nightl... / linux-job	9m 8s
test (CUDA 2.2.2, linux.g5.12xlarge.nvidia.gpu, torch==2.2.2 "numpy<2" , cuda, 12.1) / linux-job	19m 59s
test (CUDA 2.3, linux.g5.12xlarge.nvidia.gpu, torch==2.3.0, cuda, 12.1) / linux-job	28m 5s
test (CUDA 2.4, linux.g5.12xlarge.nvidia.gpu, torch==2.4.0, cuda, 12.1) / linux-job	36m 14s
test (CUDA Nightly, linux.g5.12xlarge.nvidia.gpu, --pre torch --index-url https://download.pytorc... / linux-job	1h 0m 37s
	2h 55m 5s

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