[quant][graphmode][refactor] insertObservers for Block (#34414)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/34414
Previously we insert observers for Graph(graph is a wrapper around a Block),
this PR added insertObservers for Block, so that the code can work for the nodes that have sub blocks.
Test Plan:
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Imported from OSS
Differential Revision: D20519604
fbshipit-source-id: 1908913ea7f0898cd7b4f2edd1f81cdfedf8a211
