Graph rewriting step

This would be used to remove the [node] -> Extract -> GpuImage nodes and just leave [node] behind.

Also Clone and Cache node insertion dynamically so we can utilize caching everywhere instead of in the places they are manually inserted within the document graph implementations for certain nodes.

Start by locating the document graph implementations that manually insert Clone and Cache nodes, then trace how Extract and GpuImage nodes are represented and connected. Done should include graph rewriting that removes the specified Extract-to-GpuImage sequence and dynamically inserts Clone and Cache nodes wherever caching is needed, but the issue does not name tests or files to verify this.

索引モデルが issue の本文から書いたものです。