Change cos_sim to use CosSimLayer layer when size=1 and rename convex_comb_layer to linear_comb_layer

Also add an unittest for layers.py, currently for a few things. More need to be added later.

DataProviderWrapperConverter with DataProviderConverter.

Change-Id: I6ba3ec1daeb3d4aee2556959e87bae7c4c1ad668

ISSUE=4619881 

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Thanks to caoying to check english gramma.

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