[bin] meshroom_batch: Save the graph once it has been all set up and resolved

[cbentejac]

Description

This PR fixes an issue in meshroom_batch which led its executions with the --submit argument to fail with the following key errors:

Invalid expression with missing key on "CameraInit_1.output" with value "{cache}/{nodeType}/{uid0}/cameraInit.sfm".
Error: 'uid0'
KeyError: 'uid0'

In order to prevent this, if the --save argument has been provided, the graph should be saved outside of the GraphModification event, once it has been set up entirely and resolved. If it is saved within the GraphModification, UIDs are not yet available when it is written on disk, so the dictonaries that should be filled with them are empty.

This does not cause any issue for the rest of the execution of meshroom_batch if the graph is to be computed locally, since the UIDs will be computed right after (but they still will never be written in the project file).

However, when submitting the graph to a renderfarm, the project file is loaded again. If it does not contain any UID:

• All the nodes will present the UidConflict compatibility issue;
• Key errors will be raised when trying to apply the links between nodes since they contain direct references to UIDs, which cannot be resolved as they are not present in the project file.