Weight normalization for RNN Cells. (#11573)

* Weight normalization for RNN Cells.

The class `WeightNormLSTMCell` is added in contrib/rnn. This
implements most of the functionality provided by `LSTMCell`
with one additional constructor argument `norm`. If set to `True`, this
will normalize the state transition and projection (if used via the
`num_proj` argument) matrices. If set to `False`, the outputs are identical
to that of `LSTMCell`.

Tests are added for checking this equivalence and correctness
of the normalization operation.

* fixed pylint formatting errors