Set batch size to 4 to prevent OOM due dynamic batch sizing (facebook…

…research#3781)

Summary:
## What does this PR do?
Fixes OOM which happens from TPUs due to dynamic batching exceed the max a single core can work with.

Pull Request resolved: facebookresearch#3781

Reviewed By: wnhsu

Differential Revision: D30327091

Pulled By: alexeib

fbshipit-source-id: 0ebe6b18329fa05d359083fa8ac54aba7b48bc53

examples/wav2vec/config/pretraining/wav2vec2_large_librivox.yaml

@@ -18,6 +18,7 @@ task:
   normalize: true
 
 dataset:
+  batch_size: 4
   num_workers: 6
   max_tokens: 1200000
   skip_invalid_size_inputs_valid_test: true