parallel_loader: Fix TPU memory leak when calling iter. #10009

Job	Run time
get-torch-commit	2s
Build XLA CUDA plugin / build	40m 52s
Build PyTorch/XLA / build	1h 8m 38s
Build PyTorch with CUDA / build	25m 0s
TPU tests	0s
GPU tests / test (benchmark_tests)	13m 36s
GPU tests / test (python_tests, xla_op1)	32m 26s
Build docs / build-docs	1m 59s
GPU tests requiring torch CUDA / test (python_tests, linux.8xlarge.nvidia.gpu)	10m 45s
GPU tests / test (python_tests, xla_op2)	9m 23s
GPU tests requiring torch CUDA / test (triton_tests, linux.g5.4xlarge.nvidia.gpu)	6m 32s
GPU tests / test (python_tests, xla_op3)	45m 9s
GPU tests / test (python_tests, torch_mp_op)	30m 15s
CPU tests / test (benchmark_tests)	8m 1s
GPU tests / test (cpp_tests, cpp_tests1)	10m 21s
CPU tests / test (python_tests, xla_op1)	13m 46s
GPU tests / test (cpp_tests, cpp_tests2)	17m 28s
CPU tests / test (python_tests, xla_op2)	4m 23s
CPU tests / test (python_tests, xla_op3)	6m 53s
CPU tests / test (python_tests, torch_mp_op)	9m 50s
CPU tests / test (cpp_tests, cpp_tests1)	4m 59s
CPU tests / test (cpp_tests, cpp_tests2)	5m 28s
	6h 5m 46s

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