Updated OpFromGraph docstring.
Raise error if the user try to use shared variable.
Fix error msg.
Remove OpFromGraph grad_depth parameter.
pep8
cache OpFromGrad sub grad computation. This remove duplicate op creation.
OpFromGraph now compile in make_thunk().
Make OpFromGraph support shared variable.
pep8
Updated class doc of OpFromGraph
Add OpFromGraph in the doc.
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Merge pull request #1683 from nouiz/opfromgraph

OpFromGraph documentation and cleanup