* fixed dealing with None values in then/else branches of Select
* generalized solution when condition is non one elemental
* fix when both branches are None
* minor corrections
* rewritten Select unit-tests; fixed for condition with [True] mask
* removed mutable default arg from build_graph, added a few more test cases with masked condition, other minor corrections
* corrected output_shape calculation when broadcasting is off
* layer tests fixed: relaxed assert for condition shape to let pass TF Select
* corrected shape calculation when condition is not elementwise equal, calculated calculation for TF, corrected calculation of shape when values are not set
* fixed a typo for Select from TF Where
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