Proposed changes

This PR fixes a crash that occurs when clearCache() is called while an asynchronous evaluation is still encoding commands.

MLX.GPU.clearCache() (which calls Memory.clearCache()) was not synchronized with the evaluation engine. If called during an active generation loop (for example: quitting the app), it would invalidate Metal resources while the command encoder was still using them, leading to an objc_retain crash in CaptureMTLComputeCommandEncoder.

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)