Change `config.encoder_ffn_dim` to `config.decoder_ffn_dim` for decoder impl
Change `config.encoder_ffn_dim` to `config.decoder_ffn_dim` for decoder. 

These typos are detected during loading flax from pytorch checkpoint where `encoder_ffn_dim` != `decoder_ffn_dim`

# What does this PR do?

This PR fix typos, these typos is critical!!!

@patrickvonplaten @patil-suraj

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