use module object id to cache the sharded modules #3591
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Summary:
# context
* TorchRec relies on `nn.Module.named_children()` to traverse the model to find the sparse modules to shard.
* In a normal case, every sparse module only appears once in the model hiearchy, i.e., it has **only one** parent module.
* However, in some corner cases, a sparse module might have multiple parent modules. This might confuse the TorchRec sharder due to its traversing logic: the same sparse module has multiple FQNs, and hence being sharded multiple times (create multiple sharded modules according to the FQNs).
{F1983896519}
# solution
* cache the sharded module with the original sparse module's object id
* when the sparse module has multiple FQNs, only the first time in the `named_children` traversing, a sharded module will be created.
# changes
* the change is protected by a KillSwitch: [enable_module_id_cache_for_dmp_shard_modules](https://www.internalfb.com/intern/justknobs/?name=pytorch%2Ftorchrec#enable_module_id_cache_for_dmp_shard_modules)
https://fb.workplace.com/groups/429376538334034/permalink/1343336826937996/
Differential Revision: D88218200
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Summary:
context
nn.Module.named_children()to traverse the model to find the sparse modules to shard.solution
named_childrentraversing, a sharded module will be created.changes
https://fb.workplace.com/groups/429376538334034/permalink/1343336826937996/
Differential Revision: D88218200