Clean up rnn nesting code and add raw_rnn method.

As part of the rnn cleanup, nest.flatten and nest.pack_sequence_as
now handle scalar structures (i.e., flatten of a scalar converts it
to a single-element list; and pack_sequence_as performs the reverse).

raw_rnn can be used to implement dynamic decoding of seq2seq models.

added minor tweaks to control flow gradient code to fix python3 errors
in case of zero-length while_loops.
Change: 127259950