Fix contrib/cudnn_rnn tests.

These tests started failing after
https://github.com/tensorflow/tensorflow/commit/4cedc8b6e738b7a188c9c091cf667bacafae44b7

variables.Variable is the 2.0-style class - where the notion of
global collections is being done away with (as per
https://github.com/tensorflow/community/pull/17) and hence
the call to sess.run(tf.global_variables_initializer())
is a no-op.

For now, switch the tests to use variable.VariableV1.
In a follow up, these tests need to be updated so that they do not rely on
global collections (i.e., do not call tf.global_variables_initializer()).

PiperOrigin-RevId: 220558354