Optimize cache key design

Design cache keys to avoid performance pitfalls and ensure reliable cache behavior. Cache keys should be lightweight, hashable, and deterministic to prevent unnecessary cache misses and memory bloat.

Key principles:

1. Avoid large objects in cache keys - Large values like images or models should not be part of the cache key as they bloat memory and slow down key comparison
2. Ensure hashability - Unhashable objects (like MODEL instances) in cache keys will cause nodes to always be considered ‘dirty’ and re-evaluated
3. Use references over values - Forward links to previous nodes rather than actual values when possible to keep keys lightweight
4. Cache expensive key computations - Store intermediate results of expensive signature calculations to avoid repeated work within execution cycles

Example of problematic cache key design:

# BAD: Large image data becomes part of cache key
cache_key = (node_id, large_image_tensor, other_inputs)

# BAD: Unhashable model object causes cache misses  
cache_key = (node_id, model_instance, inputs)

Example of optimized cache key design:

# GOOD: Use link references instead of actual values
if is_link(input_data):
    cache_key = (node_id, ("ANCESTOR", ancestor_index, socket))
else:
    cache_key = (node_id, input_data)  # Only for small, hashable data

# GOOD: Cache expensive signature computations
if node_id not in self.immediate_node_signature:
    self.immediate_node_signature[node_id] = self.compute_signature(node_id)

This approach ensures cache keys remain efficient while maintaining cache correctness, preventing both performance degradation and incorrect cache behavior.