BidirectionalSequenceLSTM op: Fixes merge_outputs

Bug: 123644584

Test: NeuralNetworksTest_static --gtest_filter=GeneratedTests.*lstm*
Test: NeuralNetworksTest_static --gtest_filter=DynamicOutputShapeTest.*lstm*

Change-Id: I8c7e69f4585d242301b22de7e20f9b262f4c231c
Merged-In: I8c7e69f4585d242301b22de7e20f9b262f4c231c
(cherry picked from commit 602f18290a3b1bd8f1277956047b5c5c995ed6ca)
16 files changed