fix: address AutoQASM variable declaration bug introduced by PR #930

Issue #, if available: See comment here: https://github.com/amazon-braket/amazon-braket-sdk-python/pull/930#discussion_r1550003912

Description of changes: Instead of deferring the variable declaration until the end, we continue to declare the variable at the time of assignment and then remove the declaration if it is no longer needed (i.e., when the variable is being used as output).

Testing done: tox passes.

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amazon-braket / amazon-braket-sdk-python