Set self.input_space for FlattenerLayer too.

Signed-off-by: Axel Davy <axel.davy@ens.fr>
Implements get_input_space for FlattenerLayer to avoid confusion.
FlattenerLayer: Remove redundant field.
Merge pull request #845 from axeldavy/FlattenerLayerFix

Set self.input_space for FlattenerLayer too.