Junru Shao created MXNET-790:
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Summary: Correct misusage of `CutSubgraph` in 3 control flow
operations
Key: MXNET-790
URL: https://issues.apache.org/jira/browse/MXNET-790
Project: Apache MXNet
Issue Type: Improvement
Reporter: Junru Shao
This PR corrects the usage of `CutSubgraph`.
`CutSubgraph` consumes a Symbol `s`, and
1) Assumes the attribute `__subgraph_name__` of `s->outputs[0].node` is the
name of the subgraph
2) Assumes the boundary of this subgraph be nodes who has at least one
immediate predecessor whose attribute `__subgraph_name__` is not the name of
the subgraph
3) Duplicates and puts these immediate predecessors into this subgraph
Calling `CutSubgraph` without very special care will cause disastrous
consequences, often when `s->outputa[0].node` has an improper name. For
example, in control flow operators, if we write a function `lambda: x` which
captures an outside symbol `x` (so the attribute of `x` is not desirable), and
an operator passes `x` into `CutGraph` as the first output of subgraph, the
entire procedure breaks. Without copying and manually correcting such symbols,
this will happen anywhere and be very hard to debug. An example that causes
this bug could be https://github.com/dmlc/gluon-nlp/pull/233 in which the
`pred` I pass to `cond` is calculated outside the subgraph.
This bug affects all three control flow operators, `foreach`, `while_loop` and
`cond`.
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