Fix memory leak in transforms (#2841)

[harsh-sutariya]

Proposed changes

Fixes memory leak in function transforms when a container is updated with a tracer and that container element is reused as input (issue #2841).

Root cause: When vjp()/jvp() create tracer copies and a container stores the tracer, subsequent calls create a chain copy(copy(copy(...))) that grows unboundedly, this is what prevents the garbage collection.

Solution: Flatten copy chains from previous (completed) tracing calls by traversing to the original source before creating new tracers. Only applies to non active tracers to preserve nested transform behavior.

Changes:

• Modified vjp() and jvp() in transforms.cpp to flatten copy chains
• Added test test_grad_with_container_reuse to verify fix

Follow the discussion in issue #2841 page for more details about the problem and the solution.

Checklist

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• I have read the CONTRIBUTING document
• I have run pre-commit run --all-files to format my code / installed pre-commit prior to committing changes
• I have added tests that prove my fix is effective or that my feature works
• I have updated the necessary documentation (if needed)