[BUGFIX] Free up executor memory when reassigning new executor

[access2rohit]

Description

Fixes memory leak caused due to not freeing up of graph executor object.

Checklist

Essentials

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[mseth10]

Looks like we only instantiate the object the first time because we want to check whether subgraph property is set, which has nothing to do with the executor really. We can define a variable like this and use it in the condition: https://github.com/apache/incubator-mxnet/blob/07e65e5cf3c629c78f206798f33a3529e95c0fc4/src/executor/graph_executor.cc#L58
This will simplify the code and then we only need to instantiate object once after the if block. WDYT? @access2rohit

[access2rohit]

@mseth10 Can you review again ?

[mseth10]

we should also include the check backend_name == "None" or we can do something like this before the condition https://github.com/apache/incubator-mxnet/blob/3b0b9dc9d865ee0b1456e8f400f6f4a25c77b5db/src/executor/graph_executor.cc#L59-L60

[samskalicky]

I prefer we dont duplicate code/functionality if at all possible. Since we create exec with new, and then use it to get the backend_name, why dont we just call delete on exec before line 2080 when creating a new executor from the partitioned symbol?

[mseth10]

yeah I agree, we can just use delete exec for both functions

[mseth10]

We also need to make this change for the SimpleBind API, it is implemented in a similar fashion https://github.com/apache/incubator-mxnet/blob/3b0b9dc9d865ee0b1456e8f400f6f4a25c77b5db/src/executor/graph_executor.cc#L2009

[mseth10]

LGTM

[samskalicky]

Thanks for the delete change, this looks very succinct!