[Placeholder] Fix a differentiation bug related to Placeholders.

[nadavrot]

Description:

Fix a differentiation bug related to Placeholders. The line assumed that
the destination is a variable.

Testing:

This bug was exposed in PR #1622. I did not add a specific test for this because as soon as we migrate more tests to using Placeholders this code will be tested.

Documentation: None

[artemrakhov-glow]

Should we get rid of getVariable()?

[nadavrot]

@artemrakhov Yes, but we can't get rid of getVariable until we get rid of Variables. :)
I am taking small steps right now because soon we'll need to rewrite ALL of our unit tests to using Placeholder, instead of Variables, and this will be a huge change, and I am trying to prepare for this big change with small preparation changes. In this PR: #1622 I tested the porting of MNIST. Things worked but I ran into a few things that I am now fixing. After migrating everything from Variable to Placeholder we'll be able to delete Storage and just keep Placeholder and Constant.