You can monitor gradient sparsity in QLoRA training by counting zero elements in gradients during on_after_backward hook in PyTorch Lightning.
Here is the code snippet you can refer to:

In the above code we are using the following key strategies:
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Uses on_after_backward to access gradients post-backward pass. 
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Computes sparsity as ratio of zero-valued gradient elements. 
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Logs grad_sparsity per training step for analysis. 
Hence, gradient sparsity tracking in QLoRA provides insights into parameter efficiency and optimization dynamics during training.