DropoutLayer: Use nonsymbolic shape for dropout mask if possible
Impl now requires an input_layer with a 'get_output_shape' method.
Fix brittleness funny.
Merge pull request #1 from dnouri/dropout-mask-shape

Impl now requires an input_layer with a 'get_output_shape' method.
Merge pull request #80 from f0k/dropout-mask-shape

DropoutLayer: Use nonsymbolic shape for dropout mask if possible