Fix Bug in Flax-Speech-Encoder-Decoder Test
This PR fixes a bug in a Flax-Speech-Encoder-Decoder test that was failing after push (https://github.com/huggingface/transformers/runs/5493110105?check_suite_focus=true). Specifically, it amends the `test_freeze_feature_encoder` test to **omit** the random `decoder_attention_mask` from the input arguments of the speech-encoder-decoder model. This random `decoder_attention_mask` was resulting in `nan` values on the output logits of the `FlaxWav2Vec2BartModelTest`. Removing it as an input results in real valued output logits, and the test of concern passes following this change. The behaviour of the FlaxBartForCausalLM model to output `nan` values given a random `decoder_attention_mask` has been noted.
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