[BugFix] Fix serialization of stacks of Tensorclasses #2608

# to view logs

Job	Run time
generate-matrix / generate	4s
pytorch/tensordict (pytorch/tensordict, test/smoke_test.py, .github/scripts/install-deps-smoke-te... / wheel-py3_9-cpu	1m 13s
pytorch/tensordict (pytorch/tensordict, test/smoke_test.py, .github/scripts/install-deps-smoke-te... / upload / wheel-py3_9-cpu	20s
	1m 37s

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