[tf_agents] Fix gradients in eager_utils.
[tf_agents] Fix gradients in eager_utils.

In Python 3, zip returns an iterator. If summarize_gradients is true, grads_and_vars iterator will be exhausted by add_gradients_summaries and then becomes empty when optimizer.apply_gradients is called.

PiperOrigin-RevId: 323666966
Change-Id: I83e57975a1cd7fa091fbfb06bab76cf825455272

tensorflow/agents,tensorflow/agents