Move tensor_flatten/unflatten from AQT -> TensorAOBaseClass #7555

Job	Run time
test (CPU 2.3, linux.4xlarge, torch==2.3.0 --index-url https://download.pytorch.org/whl/cpu, cpu) / linux-job	8m 2s
test (CPU 2.4, linux.4xlarge, torch==2.4.0 --index-url https://download.pytorch.org/whl/cpu, cpu) / linux-job	9m 52s
test (CPU 2.5.1, linux.4xlarge, torch==2.5.1 --index-url https://download.pytorch.org/whl/cpu, cpu) / linux-job	11m 39s
test (CUDA 2.3, linux.g5.12xlarge.nvidia.gpu, torch==2.3.0, cuda, 12.1) / linux-job	28m 19s
test (CUDA 2.4, linux.g5.12xlarge.nvidia.gpu, torch==2.4.0, cuda, 12.1) / linux-job	28m 19s
test (CUDA 2.5.1, linux.g5.12xlarge.nvidia.gpu, torch==2.5.1 --index-url https://download.pytorch... / linux-job	28m 19s
test-nightly (CPU Nightly, linux.4xlarge, --pre torch==2.7.0.dev20250122 --index-url https://down... / linux-job	11m 33s
test-nightly (CUDA Nightly, linux.g5.12xlarge.nvidia.gpu, --pre torch==2.7.0.dev20250122 --index-... / linux-job	28m 6s
	2h 34m 9s

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