⚡ Bolt: Optimize DenseLayer backward pass by taking ownership of cached inputs

[teerthsharma]

💡 What: Replaced .as_ref().expect(...).clone() with .take().expect(...) for last_z and last_input in DenseLayer::backward.
🎯 Why: Cloning these tensors during the backward pass triggered unnecessary heap allocations for tensor metadata (shape/strides). last_z and last_input are only needed once per backward pass, so ownership can be safely taken.
📊 Impact: Eliminates two metadata heap allocations and Rc increments per backward pass per layer.
🔬 Measurement: Verified by running the core test suite.

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PR created automatically by Jules for task 3099305425076104399 started by @teerthsharma

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